Is it possible that the reason it is not being released on blu-ray is because the picture quality cannot physically be enhanced? I don't know too much about this but I'm sure MPI would love to make more money.
I'm sure MPI would.
But several times I've read that, as you've surmised, the quality of the master tapes can't be significantly enhanced enough to warrant releasing the original series on Blu-ray. And it also seems unlikely that the "91 Series will come out on Blu-ray because of the way it was shot, particularly with all that smoke. I've been told it would look horrendous on Blu-ray. But who knows - maybe one day they'll develop a technology to get around that...
As for a DVD release of the material on MPI's DS Resurrected VHS, that also seems unlikely because apparently all that material was released on VHS without actually getting all the clearances for it (what a surprise considering who was behind its release and how they got caught releasing other material without the proper clearances). And supposedly trying to get the necessary clearances today would be next to impossible. But, again, who knows? I'm only repeating what I've been told. From very reliable sources, yes - but I don't know it to be fact myself...