The quality of most of the free streams is crap. The paid ones should be better, although the HD stuff will probably look better on Blu-Ray Disc anyway. A lot of people don't care about quality, as seen with copied music, which usually is compressed with lossy algorithms like MP3 while it's produced in a format better than CD sound nowadays.
Maybe some guilt shit is going on when porn consumers don't see the need for HD in porn. It's dirty and has to look shitty, so to say. Porn producers like Belladonna, who didn't like all the blemishes seen on HD once, can fool whoever they want; they just can't deliver high-end HD. There's just more to it than taking a mediocre camcorder and hire some whores and a minimal crew. Almost everything in HD porn is so obviously below the Hollywood major league or B-movies it's just natural for whores to delude themselves.
The whole push to HD will take several years for a majority of TV sets to be able to actually benefit from the format, not just some low-end "Full HD" bullshit without anything close to color accuracy and sharpness. And the free file hosting sites and torrents probably are more popular than the pay sites as well.
The low-end gonzo productions won't benefit from HD too much and therefore consumers who care about image quality a bit might just not see the point in buying a Blu-Ray Disc in this very popular market segment. Companies like Vivid, Wicked Pictures and Digital Playground seem to try to make fancier stuff that can look better on Blu-Ray Disc and will probably attract some of the pro quality crowd eventually. In a few years down the road, when more and more people realize what a crude format DVD is, and its ripped dervivatives are, it should make an impact in favor of the companies which deliver the better material. The low-end circus act league will then become a smaller part of the market and probably a niche eventually. Some call it consolidation.