I've read most of the thread and noticed that no one else has brought this up:

Dark skies.

Siting an optical (or IR) scope, as I understand it, requires a number of considerations:

Remoteness from light pollution; i.e., dark skies
Steadiness of air
Dryness of air (especially important with IR)
Minimized temperature extremes
Construction logistics

Mountains are preferred, obviously, to most easily satisfy the second through fourth conditions. Sadly, there are few left in the US that can also satisfy the first and last... and that's before the impact statements are filed and the fun begins.

Penn State, who does some kick-ass astronomy and astrophysics, had to team up with UT and build their scope in Texas -- the skies are neither dark nor dry enough in Pennsylvania any more.