Usually cause you need an established band with many songs, a selling appeal, and different sounds so they dont all sound the same (see Nickelback, I guess, though personally I like them).
Now of course you could say "hey man, like, every band has songs that sound different. you just listen to that mainstream stuff on the radio that all sounds the same, and usually isnt even what the band really sounds like brah!" (i.e. Tool. Listen to their stuff on the CDs that dont make the radio and you see what I mean). TO which I say... well, yeah, duh? thats how you get into the band by hearing it from somewhere else. And while it may be true, remember, it has to reach enough people that the game will sell well. More people Like Metallica than the ones that dont, and thus profit. Does a band like say, Tool have enough to carry a Rock Band: Tool game? I somehow doubt it (but then again the only Tool song I can say with any certainty that I remember is Sober, and its one of a few they play on the radio [at least in my area]).
...
At least the reason i choose to believe why there was never a Rock Band: Alice in Chains edition (the bastards at harmonix).
