Same as Google IO, these events have become like popular concerts- people leap on tickets because they know they're going to sell out, not because they've thought through why they'd want them.
I've been told the reason the event has been kept small is to maintain the same ratio of Apple developers to attendees. But I think that overstates the value of the Apple devs a bit. The attendees at this event really are one of a kind.
But, like another poster said, selling out in under 2 minutes makes them look good.
In my experience the value of WWDC is getting to spend time doing Q+A with Apple engineers, and moving to a bigger venue with more competition for engineers reduces the overall value of the conference rather than increasing it.
I'll give you a moment to let the sheer stupidity of your suggestion sink in.