My wife works for a lot of conventions as a Special Guest liaison and recruiter so I asked her.
She said convention sites are originally chosen generally just in the location of whoever starts it. Usually the closest big city.
Once it grows its chosen by demographic and how many people that are interested in whatever the convention is about live by. If 2 cities have similar demographics they choose by convention center. A lot of stuff goes in to this such as price/size comparison, but most importantly is their reputation. Special Guests go to a lot of conventions and learn which are good and which are bad. If a place has a base reputation Special Guests won’t go their and tickets won’t sell.
Hope this helps!
Edit: She said the Washington State Convention Center where they hold PAX is easily the best