As far as I know PS was allways built with Mac hardware in mind. Their software is optimized for MacOS and meny of their technologys (PDF, true type, etc.) are incorporated in the Mac much better than any other computer. Apple and Adobe have allways had a sort of symbiotic relationship making the two almost sinonymous.
The porting of PS and their other software to the PC was surely just to tap in to the cash that market offerd. If you ever use PS on the PC then try it's mac counterpart you will see what I mean, don't trust numbers or any other hype, running it on a Mac especially PS7 on OS X, is quite impressive.
But as far as linux, I think it's just the market. It's hard for a comercial computer company to see a nich market full of rebel computer nerds and want to develop for it. I'm not saying this is good, infact I think it sux, because attracting big companys is what Linux is going to have to do in order to grow.
Maybe pettitioning Adobe would convince them to release a port.