In the pre-home media days, Disney re-released movies to theaters on their X Power of Ten Anniversary. Additionally, up until 1990, you'd wait years and years for Disney films to arrive on video. (Some studios/directors had weird, weird attitudes towards VHS. Anyone remember when "E.T." was originally "never ever" going to be on video?)
"Little Mermaid" was, and this was a huge shock back then, put out on VHS a few months after it's theater run. That helped make it still possibly the most popular Disney animated film ever. You may notice that whenever "Mermaid" or (most) other Bronze Age Disney films are sent out on DVD, it's extremely rare and they are vaulted almost immediately. Meanwhile, we're on our fourth or fifth DVD of "Dumbo". So basically, the vaulting "math" might go like this:
Most Popular Disney Movies - Rarely released on DVD, big occasion when they are. ("Lion King", "Fantasia", "Sleeping Beauty", "Mary Poppins", ect.)
Moderately Popular - Released very often. Not much of a marketing push for them. ("Dumbo", "Alice", ect.)
Unpopular - Ffft, good luck. These were put out on DVD once and haven't been seen since. (Anthology films, "So Dear to My Heart", "Victory Through Air Power", "Emporer's New Groove", "Atlantis", ect.)
Cold Day in Hell - "Song of the South". I... may have a big, long essay about this coming up on my blog soon so sit tight for that. For now, if you want to see what the big hubbub is all about, you'll only be able to find this on bootleg.