It's because of their promotion style. They are the first kpop group to try and be super active on SNS at an international level. And because of this constant interaction with intl fans and huge amounts of content they provided they were able to solidify a big international fanbase before even making it in Korea.
I think it's because they are more free to socialize with their fans on twitter also they are talented and have the best personalities on k pop, no shade to anyone but they are funny as hell and really care about their fans I mean that's the reason why I started becoming a fan, they make me laugh.
I think EXO has great songs but SM fails into let them be themselves and its pretty clear that they dont have much freedom and you can see that they are pretty indiferent with each other and shade themselves alot from what some people have told me I mean someone told me that Chanyeol remarked that the merch that doesn't sell alot was the ones which had Suho on it and Kai doesn't like Chanyeol too lol.
And the music is great I mean Butterfly, Young Forever, BS&T and others are so great, I can see them and BLACKPINK breaking into the US market soon.
Broadcast in dozens of English speaking countries to millions of people
I love 2NE1 but it's pretty deluded to say this, ARMIES are literally the most popular and big fanbase right now, they stole practically 1Ds fans, wherever you go you will see their fans commenting, 2NE1 would be big too if they debuted in this era like BP.