Actually, NPC and the NIC are both in Indy!
Also the Association of Fraternity Advisors [AFA] (the professional organization for Greek advisors) was in Indy. They just moved last week down the road to Carmel.
NPHC is headquartered in Indiana also at IU-Bloomington; as well as the Center for the Study of the American College Fraternity, and Lambda 10.
I think the main reason is definately the tax breaks, but also it is centrally located. Their is a large airport, and it's easy to get in and out of for Headquarter's Staff, Consultant's, etc.
Of course, I think there is plenty of things to keep you busy socially also.
It's no Chicago, but it is a fun city still the same!
o.p.i.e.