I kinda answered that. In the early days, Andy never got signed releases from the people appearing in his films. This came back to haunt him after he released Chelsea Girls because it was one of the few profitable films Warhol released. Mary Wornov's mother sued Warhol when Chelsea Girls was released because Mary never signed a release and she didn't get paid to appear in the film. Warhol ended up having to pay the cast which is something he typically avoided doing.
The same thing would happen if the estate released any of the early films on DVD today. By law, someone can't film you, release it and make money off of it without your expressed permission. That's the point of the release - you sign away your rights to try to get future monies from the film (or whatever). I think Warhol's estate is worth something like $600 million, but I can't imagine the estate is generous enough to just release the films on DVD for free. Even if they did, they'd still likely run into trouble from someone.
You have to remember, a lot of what is featured in Andy's film is not....positive stuff. People shooting drugs, having sex, etc. If you're an ex-Factory star (or even a hanger on or whatever) and you're still alive, pushing 70 years old and living a nice quiet life, odds are pretty high you don't want some people watching you do stupid stuff you did 40 plus years ago when you were high on speed!

Those films weren't intended for general release which is why a lot of people did the stuff they did - they never imagined anyone would see it 50 years later! That alone would probably cause a few people to sue to stop the release.
Like I said, most of the films are....around.

I've seen the ones with Edie (Poor Little Rich Girl, Beauty #2, Kitchen) and they're interesting to a point because it's Edie all her glory but honestly, they're mostly terrible. I'm a huge Edie fan but even I wonder how those films brought her any kind of fame. I guess it doesn't help that I watched them sober!