It's mostly lack of funding. Due to Title IX, schools have to give equal funding to sports for both guys and girls sports teams. A lot of sports are seem as more of a male thing. since schools have to spend the same for both guys and girls sports, the disparity in the number of guy sports vs. the number of guy and girl sports means alot of physical, more masculine sports have been axed over the last couple of decades in order to comply with Title IX. Couple in how much basketball and football get in funding and there's often not very much left.
As far as wrestling goes, its mostly larger schools that can spare the funds to give any sort of support for it. I don't know about the rest of the country, but in south Georgia the wrestling teams that exist often resort to using the football equipment. I remember doing alot of fund raisers so we could pay for going to the tournaments(the money the high school gave us for the season would be gone after 3 or 4).