Voice and themes of his music.
I think another way to look at it is by asking the question in reverse. Why should he have blown up? The beats are cool but nothing really stands out. He didn't really bring in a new sound/style production wise. What are the main themes of music? One minute he's a pimp, the next he's a loverboy, the next he's a political commentator, the next he's an advocate for everything southern. Then, every album is more or less the same formula. At some point we were pretty much getting the same album over and over except the previous version was slightly better.
He doesn't have a clear story or movement that people can champion. He's a talented rapper who knows how to make cool beats.
Also another thing, his lyrics are kind of a downer. His message is always "im underrated, people don't respect the south, etc." that's the whole attitude of his music (at least for a casual fan). Wale is kinda like that too. If you look at Wiz, Spitta, Kendrick, ASAP, Drake, Cole, Dom, YG, Tyler, etc, you don't really get that vibe. It's not "i should be more popular than i am, I'm too slept on" vibes.