Back in August, there was a thread on suggestions for improving The National.
http://www.net54baseball.com/showthread.php?t=114661
Mike Berkus was kind enough to come on and give some of the thoughts and logic behind their planning. If you don't have time to review the whole thread, he made some key points in posts #79 and #115.
The first factor they have to consider is available floor space at a convention center. They shoot for a
minimum of 300,000 sq. ft. Baltimore will be the tightest ever with 290,000 sq. ft. I think Cleveland has over 500,000 sq. ft. available.
Very few cities offer this much space
on one floor, in one location.
After that, you get into things like set up costs (This eliminates NYC.) Then you look at hotel rooms available, transportation, etc.
This thread explains why many cities aren't even considered.