Its all about brands. (Score 2, Insightful)

A small game development company/indie developer cannot afford all this publicity and ads. Not to mention they are unknown and new to begin with. A large company with a recognized brand will get more buyers and will spend much less effort to launch a new game. If Indie Game developers would cooperate on advertising and concentrate their games under single brand people would recognize and buy these games.(Yeah, they wouldn't be "Indie" anymore in the strict meaning of term.)