A lot of it is simply budgetary - Pitt still has a great reputation and a great program for security studies, but Pittsburgh itself (the city) is cheaper and the program is less expensive as well, and I think I'll have a better shot at getting funding there. In addition, I spoke with the head of admissions at Pitt at an APSIA grad school fair and he gave me a really great impression of the school. To be honest, there's not a large degree of difference between any of the programs on my list (with the exception of American since their program doesn't have as strong a focus on intell), so I'll be happy with whichever programs I can get it into (especially with funding).