It's not so much specializing in a specific submission as it is having a game that provides you with chances to go for a specific sub over and over. People don't give you shit at the higher levels of MMA (or at least, it's rare), so you have to create your chances and learning a whole game to create chances for more than one type of sub is hard. Palhares is really good at those leg entanglement entries, so he built a grappling game around that. The heel hook just happens to be the finish. Ronda had already built a game around attacking the arm bar from Judo, so she just stuck with that. All the TAM guys are pretty good wrestlers so they end up in front headlock positions a lot, so they optimized their finishing chances from there by getting really good at guillotines. Maia is all about takedown with continuous control and minimizing damage and counterplay, so taking the back is perfect for his style and he built a game around getting there. Don't concentrate on how someone finishes, concentrate on all the things they do to get to that point where they can finish. I wouldn't say the finish is incidental but it's not the hardest part.
If you look at people who catch a lot of subs against lower level people but get stymied against high level guys (Joe Lauzon and Charles Oliveira come to mind), a common thread is that they throw a lot of subs but don't really have a systematic setup game. That works against lower level guys who make more mistakes and give you more chances, not so much against the top of the heap.