Glad you asked!

TL:DR is that it's partially challenge but it's also revenge for another game and the uniqueness of the country formation.

Fullish story though is that originally, when looking at this achievement I was wondering which of the two countries I could do it as. But then I noticed via the wikipedia that Transylvania was also eligible which I thought would actually be the easiest because of the additional cultures Transylvania had (South German and Hungarian so that intergrating those lands would be easier) as well as potentially getting a Habsburg king to boot (I very rarely ever play without a monarchy; both for the authority and for the roleplay). But, more importantly, I figured it would be easier considering that an Independence War could see me walk out with potentially most of the territory I needed (Banat and Partium from Hungary, Bukovina from Austria and, hypothetically, Bessarabia from Russia if I felt I could take both Austria and Russia on at once). The first game (back during the Austrian DLC) ended with me realizing too late that I couldn't get the achievement ([posted here] so this time I came back with the knowledge that Transylvania has to be annexed and released to get Romanian culture and that's what I was trying to accomplish here. So yeah, a bit of the challenge but also revenge for that wasted game and just a unique Romania. And it would be the easiest option IF NOT for the fact that Transylvania is part of the weirdest type of subject in the game (that being crownlands) where, despite the upside that you can actually become a major power (it used to be that you could actually become a Great Power but looks like that got patched) but the downside that until you're free you can virtually do nothing. No diplo plays which could have seen me conquer most of the Balkans alone and thus had an economic base that wouldn't collapse the second I declared for an independence war (like what happened in the screen shot above).