it's not often that all three companies are desperate to prove themselves.
1. nintendo needs the wii u to finally be considered worth buying
2. sony's seeking a comeback from the ps3 generation
3. microsoft has to make sure people get hyped up on the xbox one after a disastrous reveal
...so everyone's hungry. and as the generation settles in, there are usually less things to be excited about.
but aside from that, big conventions are becoming increasingly irrelevant when companies can effectively market to their own crowd without the need to design booths, create demos, set time aside for conferences, etc. i'm not sure that if we get to another generation, that it would be as exciting as this one.