I think it really depends on the reasons why they left... which is probably something only they *really* know. I say this, because unless there was some huge falling out and everyone witnessed something that made them want to leave, all you really have to go off of was why they *told* you they left. It could be they left because they wanted something from a sisterhood that they didn't see ... or because something bad happened...
I was once a member of a local sorority, as were many of my close friends from the first university I went to. A lot of baaaaaaad stuff happened, and about half of the local sorority deactivated... myself included.
Some went on to join the new KDChi colony that was forming (which I believe is predominately a latina sorority), and the others (myself included) started another local with the intent of going national. I transferred before they colonized with a NPC group and found my home in ADPi up here. They are now all happily initiated members of another NPC at my old university.
The school was not a very big greek school... at the time all this happened there were only 2 local sororities, and the KDChi colony. One of the local sororities went national w/ a NPC group, the other local (that had many members leave) stayed local, and the group that broke off to form the new local eventually went on to becoming another NPC.... so maybe your perspective depends on the school atmosphere....
Personally, I think it's sad to see members leave, but ultimately, everyone -- the local, the new local-turned-NPC group, and myself are all happy with the way things turned out. Sometimes things happen for the best.
All you can do is wish them well and move on with your own chapter.