I wouldn't call Tri-Sigma or Phi Mu "big" nationals, they're more medium sized (that is not a slam, just a fact - I am in a small national) so those really aren't groups to judge by. Chi O, ZTA and DZ have the largest number of chapters.
The fact is that Delta Gamma has had chapters there for a long time, so they have lots of alumnae, so if they want to start a new chapter there they have lots of support. My sorority only has one alum chapter there and I think most of them are over 60. It's not that we don't want to go there, it's that we wouldn't have much alum support or name recognition.
Here was a discussion on this:
http://www.greekchat.com/gcforums/sh...threadid=29876