Before steam started supporting macs (what 2 years ago or so?), the options tended to be games ported over a couple of years late being sold at $50 while the PC versions cost about $20. The big exception being blizzard and maybe a few other companies.
I mostly just stuck to consoles with a few exceptions for that reason.
Once steam started supporting mac it was like a treasure trove of awesome titles at awesome prices as companies (and most of it is still indies) realized hey maybe people will actually buy this if we're reasonable about it.
Edit: Apple's general popularity going up around the same time as steam supporting macs likely helped with brand awareness as well.
Double Edit: And please no mac-pc debates now, those make me so sick