Sort of a chicken or the egg thing. If NYC had a top program, I'm very confident they could get NYC talent to stay, but that hasn't existed for decades at this point, and getting back to that level feels incredibly unlikely. St. John's has, at best, mediocre facilities, a mediocre campus (with significant space constraints if they ever did want to build out), and doesn't have a rabid fanbase.
NYC is a pro sports town filled with transplants with allegiances to other universities. I spent many years living in the city, including a lot of time visiting friends that grew up and lived 15 minutes from St. John's in Queens and I have never once heard anyone tell me they were going to go catch a St. John's game (or met any fans of St. John's other than alums). There's for sure 10+ universities that have better fan support in NYC than St John's (us included).