Expansion is a lengthy process, which is why Delta Zeta chapters aren't popping up everywhere quickly. However, this year we will be getting 3 new chapters which is a really exciting thing!
Expansion depends on a lot of things: whether the campus is interested in having another NPC chapter, whether DZ is interested in colonizing at that campus, what other NPC organizations might also be interested in colonizing... The list goes on. In the end, I think our national officers do a really good job of selecting campuses that offer DZ the chance to thrive. It's a slow and expensive process, but, in the end, it's well worth the effort.