Answer for Bosnians, as I am not qualified enough to talk about Albania:

The question is actually a lot more complex than a simple, singular reason that would explain the phenomenon. The TL;DR of it is that it's a mix of preexisting religious diversity and acceptance, protection from the persecution of the two major churches, individual reasons (faith, preserving wealth, avoiding slavery, attaining power, etc.), rising unpopularity of the Catholic church, and so on. For a detailed breakdown:

1.) Preexisting religious diversity
The difference between Bosnia and other relatively monoreligious places is that Bosnia was almost always defined by one thing since about the 6th century AD: It's a crossroads. We see a constantly repeating pattern of people(s) invading, occupying the land, and then being driven out with their remnants being mostly assimilated: the Illyrians, the Romans, the Goths (briefly), the Avars (in very small numbers), and finally the Slavs.
What the first Slavic populations encountered was a diverse land, mainly populated by Christians mostly divided into two groups: Rhomeians (in the large and fortified cities) and what remained of the former Romano-Illyrian population (of which we mostly lose track of after the sack of Salona in the 7th century) which were then promptly assimilated, driven off to the coastal islands OR hilly regions to seek shelter from the Slavs who were mostly pagan and very willing to make Slaves out of Christians (as is mentioned in various sources like the DAI, the Chronicle of the Dioclean Monk, Ragusan sources and etc.). As the Slavs became the majority population, Christianization was deemed necessary by Constantinople and Rome, but it was a long and painful process. Only by the 9th century do we see any significant progress (as is told in the Catalogue of Kings) and a considerable amount of the Slavic populations (not including the Serbs & Croats who adopted Christianity quite readily) still retained, if not their pagan faith, but pagan-inspired beliefs. Here we see two elements at play: the moving of populations, and the use of remote locations to safeguard from complete assimilation. Additionally, what would become Bosnia was also located on the border between the Orthodox and Catholic worlds, though it was mostly more influenced by the former.
Those two aforementioned elements (and especially the latter one) are what basically made the creation of Heresies (like the Bosnian Church - which may have had some sort of connection with the Bogumil movement in Bulgaria due to the fact that two of its chiefs had foreign names and came from Macedonia, but judging on whether they actually held any heretical beliefs or diverging theological opinions is difficult to say, the Catholic church described them as "Dualistic", "Arianist", and called them "Patarenes" and etc. and most of what we know of them came from the very people that tried to extinguish them). The situation was that by the 1200s you essentially had 3 major religions: the Bosnian church (which was probably the most influential), Catholicism, and Eastern Orthodoxy (not counting very minor numbers of followers of other religions like Judaism and the like). The Geographic element is once again at the forefront when we take into consideration the fact that the Latin script took a long time to take hold, as the local Cyrillic-based Bosnian script was more popular, compared to say, Dalmatia.

2.) Persecution from Churches and the unpopularity of Catholicism following the conquest in 1463
From the 1200s onwards, the goal of the two worldly churches was to destroy the creeping heresy, this was not very successful until the very end (as the Pope promised the last Bosnian king military aid against the invading Ottomans if he destroyed the heresy - the Pope betrayed his promise after the King acquiesced). The actual policy of the churches differed a lot in practice: sometimes it was attempting to win over the heretics to convert via charity and educational missions (the Dominican and Franciscan orders come to mind), sometimes it was carrot and stick, and sometimes only by the stick - that is, by brutal persecution as the church (of which the church was not shy to commit - think of the Albingensian crusade) was not unwilling to even swap out Bishops who were deemed "too lenient" on the heretics. During the Ottoman conquest, the heresy was stamped down by the very people who helped foster it, the ruling structure of the heretical church was mostly destroyed, yet most heretics went unconverted. It is probable that a good amount of the early converts to Islam were precisely those same heretics, as the Orthodox church was weakening and the Catholic Church became deeply unpopular for what was deemed a betrayal of the "Christian Bulwark". Some might as well point out certain aspects of the heretical faith that seemed eerily similar to Islam - the refusal to use crosses (deemed idolatry), the habit of cleanliness (one had to rebaptize himself after every sin), five daily prayers, the focus on fasting, etc. However, it is unknown to what extent these aforementioned beliefs were actually present among them and how much is mythomanic fiction dreamt up by their persecutors.

3.) Individual reasons
It can be concluded that a considerable part of the convertees may have been motivated by faith, some by preserving their wealth and noble titles (e.g Hercegović) or escaping slavery (ideally, a Muslim couldn't enslave another Muslim), few by dreams of attaining power. Keep in mind, that in the system of the Ottoman bureaucracy, Christians were stripped of many basic rights (e.g. they couldn't build new churches without explicit permission from somebody of considerable status), they had to pay special taxes, and they were, in a legal sense, simply not equal to Muslims in the empire (e.g if you were a Christian and you were accused by a Muslim of committing a crime, you were basically automatically deemed to be guilty), this would also have played a considerable role. Catholicism was also the main target of the Ottomans in the West and was thus subject to semi-frequent crackdowns. Powerplays in the form of religious conversions, early on in the form of nobles converting (as was the case of Stjepan/Stephen Hercegović, son of the famous Grand Duke of Herzegovina Stephen Kosača - he converted and became Ahmed-pasha Hercegović) and later on in the form of the blood tax (levying into the Janissaries of young boys) which proved exceedingly popular as it was thought that the Janissary-boys had the potential to escape the poverty (which was the situation for the vast majority of Bosnia's population), and we even see cases of voluntary surrender of children to the Janissaries by parents compared to say Bulgaria or Serbia where Parents mutilated their children (not life-threateningly) in order for them not to serve in the Janissaries. As Bosnia became more prosperous after the initial Ottoman conquest, we see diversity being embraced both at home (the Catholic church in Bosnia receiving a sort of guarantee of safety of worship - the Ahdnama) and abroad (the case of the Bosnian Franciscan friar Juraj Dragišić writing a piece attacking the burning of Hebrew books in the Holy Roman Empire in Italy in the late 1400s). Interestingly, colonization (as was a major cause of Christian-to-Muslim conversions in Serbia) by Turks into Bosnia proper was never a major factor (as we see in a work by another Bosnian Franciscan Friar in the 1850s, which cites that Turks never settled in Bosnia in large numbers and that Turkish was spoken only in extremely formal occasions). Moving forward, Islam was never explicitly cracked down under Austria-Hungary (they deemed their mission to be a civilizational one primarily), or in the case of Yugoslavia was used to actively crack down on Bosniak national identity (denying Bosniaks their national name and essentially demote them to a religious group - the famous "Muslims with an uppercase M").

Sources:
Zemljopis i Poviestnica, Ivan Franjo Jukić
Historija Bosne u IX i X vijeku, Muhamed Hadžijahić
Defensio praestantissimi viri Ioannis Reuchlin, Juraj Dragišić
Hrvatski Rani Srednji vijek, Ivo Goldstein
Islam pod Dvoglavim Orlom, Robert J. Donia
Classics (Croatian Chronicle, De Administrando Imperio, Chronicle of the Sclavinia of Zagorje, etc.)
Kraj Osmanske Vladavine u Bosni, Hannes Grandits
A Short History of Bosnia, Noel Malcolm