The primary difference between PlayStation VR, and the HTC Vive and Oculus Rift, is that the former runs only for consoles – specifically for PlayStation 4 – while the latter two work with high end PCs. This works out well for the PlayStation VR, as being paired to a cheaper console makes it more of a mass market proposition than Vive or Oculus could ever be- but it also does not explain why it has no compatibility with PC in addition to its PS4 compatibility at all.
That may change some time in the future. According to Nikkei, Sony is exploring ideas to bring PlayStation VR compatibility to PCs some time in the future; Nikkei states that nothing in the short term is planned, however, and that for now, Sony’s focus seems to be squarely on the games.
If PlayStation VR does come to PCs, it would mean PC owners would have a cheaper VR solution- the PlayStation VR is only $400 to the Oculus’ $600, or the Vive’s $800. Certainly, it could expand the reach of the PlayStation VR even more.