Blu-Ray is a comparatively new tech. Like the DVD when it was newer, they're going to do new releases, then older ones they think will sell well, and eventually remaster non-HD stuff if they think it will look good.
There are movies coming out right now for the first time on DVD, I just got Student Bodies, for example, as it was just "officially" released on DVD. I previously had a bootleg obtained on ebay before they got wise. It was crap of course, but it was on DVD at least. But I wasted no time getting the remastered release. Probably too low of quality to ever look better on Blu-Ray though.