Well, they sold out ~5000 tickets in 12 hours. True, that's not small, but clearly there's more demand than they are providing for and a lot of developers got left out. It's the same size as Google I/O for comparison.
Splitting off OS X from iOS would probably help a bit but Apple won't do it.
The last couple of years, Steve has said that he's sorry he can't find a bigger venue. Perhaps that's the problem? Which venue could the conference use to support more attendees?
And then there's the problem of keeping the ratio of apple engineers constant to not lower the value of the conference (which is part simple knowledge and part direct access to Apple engineers) for attendees.