More than most, Kevin & Bean is all about local. They make fun of the other KROQ jocks. They talk about Channel 4 news people. They reference the Lakers, the Kings, the Dodgers. They talk about the entertainment biz in way that L.A. folks would understand, but the rest of country wouldn't. They center so much of their show on local events, like K&B singles parties, Weenie Roast, etc.
In other words, K&B is big because they're local, local, local. Will they strip that away once they go national? I sure hope not.
By the way, so much of their humor is also about themselves. Do you have to get rid of that when you enter a new market, and listeners don't know that Kevin's got a bad memory, Bean has weird hobbies, etc.?