Why is it that the VGAs have more reveals than E3?
Time of year and television audience (or people's prejudgments and assumptions about the size of the television audiences at both).
Time of year more than the other, though. The biggest games come out in the fall (and then half are pushed to February, these days), and the publishers want to start the hype for their games a good year in advance. Four or five months isn't quite the same, and neither is seventeen. About twelve is ideal, to them.