Why? It's just a very hyped minority interested in these frames. Like others have pointed out, there is little to no marketing done by Wilson on this "new line." It's not even that easy to get to this prolabs minisite unless you have a direct link or know exactly what you're looking for. Also, almost every single Head pro stock frame has had a retail counterpart, so if you want to play with X players frame, with some digging you can find out what to buy. Unless you want them to release retail versions of the PT346, PT113b or some of their very few non-retail pro-stocks, which is highly unlikely.
On another note, which is quite ironic coming from a H22 lover like me, I'm amazed how much of a cult following the H22 and H19 has sometimes. Especially considering the H22 was only used to win 1 Grand Slam singles title (Novak's AO title with Wilson). Then the only players in the ATP top 100 that I know that are rocking the H22 currently are Carreno Busta, Kachanov, Sandgren, and Pospisil... Then H19, the only "mainstream" players I can think of rocking it are Monfils and Keys. I guess the elusiveness and exclusiveness plays a big factor.