I think the issue is the Parking apps are targeted at a very small subset of people. the only real audience are people who occasionally park in a city. I suspect that most people in that subset rely on their hotel or destination parking suggestion and leave it at that.
I did try out several apps (and web sites) during a recent weekend trip to Boston. Several were next to useless, a couple were good. I ended up saving about 50% vs what my hotel valet service would have been - but I did have to walk a half mile from the garage to the hotel.