It's not because you can only play fake, plastic instruments. It's not because you spent all of you budget surplus on gaming instead of real instruments or music. It's not because you have no style or, more importantly, a stylist.
It's because your band name is horrible.
On an old Ricky Gervais XFM episode, he hypothosized that a good band name is one you can imagine an announcer booming out over a packed arena. Few bands on the Rock Band leaderboards have names that can fit this bill. I've identified at least three types of Rock Band names, as outlined below with examples.
The Possibly Clever but Unfunny Joke
The thing about a band name that most people seem to forget is that it's spoken, seen, and heard over and over. The kind of joke that can fit into a band name is rarely lulz-worthy and never replayable. Often Rock Band handles are just crude. Thanks for the maturity, gamerz.
Examples: Jomeo & Ruliet, Officer Winsolow & the Urkels, Barder Than Thou, GO FIST YOURSSELF, Lactating Titties, Sotally Tober!, Just Human, Sects Fiend, (Witty Band Name)
Good Band Name, Already Taken
Occassionally one will see a Rock Band name that actually works. Unfortunately it's already used by a real artist or remarkably similar (sometimes purposely) to a real band name. These are my favorites.
Examples: Swell, The Winters, Everytime We Fail, Death June, The Ted Kennedys
The majority of Rock Band names are just horrible. Maybe they're inside jokes. Maybe the 12-year olds who came up with the think they're really awesomes. Whatever the reasons, they need to stop.
Examples: X-Box Killas, The Dealaz, Swan Struck, CARIBBEAN STUD, Sins of Sorrow, Jokerz Wild
This post leads to one question: "What's your Rock Band name?" My wife and I started a band named Gun Singer. Not great, but not in offensively band taste either.