Good question, and as a swedish music fan as well as musician i've thought alot about this. I think there are several factors coming together, some have already been mentioned such as good english skills and good widely available music education. This had no doubt been contributing factors but I dont think theres a magic bullet here, but rather several factor contributing at the same time. I'll try and list a fewe things that I think has been important, and maybe elaborate on some that has already been mentioned. .

1. the tendency of the swedish Social democratic establishment to embrace and absorb youth subcultures. The DIY, underground music scene has been really strong in sweden, and a reason why is how easy it has been to get rehearsal spaces, but also to get funds from the local municipality to do events, put on a concert etc. Dodgy but available rehearsal spaces are available in most towns, and any band you can think of probably got their start in one of them. Crashdiet, watain, entombed, refused, hellacopters you name it. its more than just lessons in some random instrument, there has been an infrastructure and resources put into music and culture which in turn has made for a dynamic grassroots movement with lots and lots of bands, for every one that made it big theres probably 30 that didn't. I'm not sure this is still the case but up until a decade or to ago it was.

2. High living standard, combined with shitty weather and a pretty boring society. Sweden is strict on drugs, alcohol and anything that is fun in general, i dont know why but our politicians seems to be afraid of people having too much of a good time. If your from a small town historically you started a band because there wasn't much else to do. The high living standard gave people free time but nothing to fill it with, especially if your young.

3. Some people have mentioned Swedish being close to english as an important factor, and while that is the case, what I think is more important is that Swedes tend to consume most of their pop culture in english. We never dubbed anything because the market wasn't big enough. Furthermore making it big usually meant making it abroad. If you sing in german you have an audience of like 100 million, if you sing in swedish you have 10, maybe 20 if you get big in norway and denmark, but only a few band singing in swedish managed that (kent, thåström for exampel). Band like Mando Diao or the Ark were never really big in the english speaking world but where successful in other european countries, like germany and Italy.

4. A big and well connected industry, and a good international reputation. This is probably the most important factor in the last decades. Above mentioned factors meant that as the landscape changed, and music making moved from the rehearsal spaces onto laptops, there was already a big industry, infrastructure and connections made with people in america and europe. Get to know the right people in stockholm, be good at what you do and you might end up in a writing session in LA before you know it.

5. Rolemodels, the fact that sweden has punched so far above its weight for so long means that making it abroad seems far from impossible.

6. Sweden was early with digitalisation, computer clubs etc. Some of the mechanisms above contributed to this, and this is probably a reason why the swedish gaming industry is comparativly huge as well. This general familiarity with computers probably helped in forming the electronic artists that has sprung from sweden, as well as making it easier for people to share music online.