You would think after all the cons, they would try and find larger venues, to support more attendees. This also lends creedance to the notion that 1 blizzcon is simply not sufficient.
10 million people around the world play WoW. That number does not include the number of people who play just Diablo, or just Starcraft, or just Warcraft. What venue do you suggest that could hold just 10% of that?
Blizzcon is held minutes away from their world headquarters. With all the computers, statues, stage, sets, and tons of other awesome things for Blizzcon, the cost to move all of it elsewhere would jack up the price to the extreme (Blizzard already loses money on Blizzcon).
Blizzcon/WWI have been held in other places - typically, close to a Blizzard office. France and South Korea have both had events held. Plus, Blizzard typically does eSports events around the world.
Blizzard also frequently has a presence at many of the major cons around the country.
Finally, name another major con that ISN'T held in the same location for years and years. Gencon is always held in Indianaopolis. PAX is always held in Seattle. DragonCon is always held in Atlanta. SDCC and NYCC are always held in their respective cites. Why is Blizzcon special in that it must move?