My theory is that each of these shows has some obscure fanbase. Another theory could be that by releasing the shows with the more well-known characters (
Gilligan's Planet? Heard of that? And
Yogi's Gang?) could get them some quick bucks. Yet another theory is that since those shows are newer, they're easier to remaster.
From what I've heard, the reason more
Huckleberry Hound hasn't been released is because of music licensing issues, but you probably knew that.