"Printers both older, contemporary and newer than it all support Postscript. So why didn't it? No idea."
Because PostScript is a fully-fledged programming language. You can even render the mandelbrot set in it, *on* the printer. That costs money, because you need to put a fairly beefy (for the time) CPU in the printer.
HP PCL is simpler, but still provides for font rendering (and vector graphics?) on the printer, so you still need something relatively beefy in the printer to do that.
Manufacturers can save money by doing the rendering on the PC and sending a simpler rasterised image to the printer. Hence the custom printer drivers.
Plus, I'm assuming that Adobe charged a license fee for supporting PS (and HP for PCL), so there's that, too.