Streamlined and popular with plenty of supplements.
If a game has lasted for over thirty years, through countless editions and company changeovers, it obviously has a fanbase supporting it, and you can bet they're fans for a reason. Why has Dungeons and Dragons kept going since it was just Gary Gygax and a couple friends in his basement? Because it's good. Sure, each of the editions has its own problems (which each subsequent edition tries to fix), but the basic idea of playing Conan-esq adventurers in a Tolkein-esq world has proved time and again to be fun. Dungeons and Dragons is mainstream for a damn good reason.
Another bonus mainstream games have is versatility. Much as I love indie games, they're usually designed to do one thing and one thing only. If you're playing My Life With Master and decide you want your character to abandon his vampiric overlord and lead a revolution in the next town over, guess what? It ain't gonna happen. On the other hand, games like D&D or GURPS can cope with it. They've got lots and lots of options, and can cover just about any sort of situation your characters can blunder into.
On the other hand, this leads to the major downside of the mainstream...
If I were to make a broad and unfair generalization, I would say that a great indie game does one specific thing really well, while a great mainstream game does a bunch of things pretty well. The drawback to versatility is that without focus, all those wonderful options you've created can start seeming bland and same-y.
Mainstream games want to appeal to everyone and turn off as few people as possible. You want a game of espionage? Sure, we can do that! You want a game specifically about your spy dealing with her repressed memories? Sorry, that might bore too many customers. You want your hack and slash monster killing campaign to be psychologically complex but filled with dark humor? You'll have to rely on your players for that, because our main game books have to be a bit more broad.
Obviously, good players will add all the flavor you need for any sort of campaign. But if you want to push them in that flavorful direction, you might find it harder to do with mainstream games.