There are different selling points for different people. Mainly, a consideration is cost. Since it is funded by a grass roots movement cost is very important. Also considered is what there is to do in the area. If people bring family, there should be things for the family not attending the conference to do during the day. Things also considered are locations of Perl groups, and where other conferences are. Having different locations allows for people who go to YAPC(s) to see different places.
There hasn't been a YAPC::Mexico... yet :-)