Costs too much is the plain and simple answer. Not just the price to get in but the related travel costs, hotel rooms, etc. I don't really care about the convention scene anyway.
My wife scheduled a family vacation during GenCon!
I live in Wisconsin. I have lived here all of my life. In the 80's, when i went to my first GenCon, it was 50 minutes away from my house. In the 90's I moved closer..about 20 minutes from downtown Milwaukee.
I've gone to 16 GenCon's in my life. I've loved every one of them. Either I meet new people or play new games. I used to be a hardcore 24x7 GenCon gamer. Now I go to socialize, see new stuff and just geek out for a full 4 days.
This year, my wife decided to make the arrangements for a honest-to-goodness Clark W. Griswold family vacation to Branson, MO. It isn't such a bad thing because my usual expenses for GenCon ran about 250-300 when it was in WI. My Indiana trip was closer to 800.00 and I still haven't read through all the stuff I bought or put together all the mini's I purchased.