The HBCUs don't have the state of the art facilities, top recruiters, appealing geographical locations (most are in rural southern areas), or budgets to compete with those PWIs.
Telling a 17, 18, 19 year old kid you can come play here when they know they can go to a PWI and play in front of 100,000 fans in a stadium while their game is nationally televised with pro scouts watching from the stands and watching it on TV is tough sell.
This is not to mention those players are treated like royalty at those PWIs. Its a tough sell for kids getting courted by those schools that are consistently putting players in the league and competing in Power 5 conferences.