Equinox is noteworthy for a number of reasons.
1. Like The Blob, which Jack Harris also produced, it wasn't a major studio picture. In fact it only exists because a bunch of college kids successfully filmed and edited together a full film which, uninteresting or not, had a plot, and, poorly done or not, did feature elaborate special effects done pretty much entirely by them without any help from anyone. Nevermind that the resulting film is clunky as heck, it's a miracle it exists at all.
2. Jack Harris was able to take it, re-edit it, have new scenes filmed and put in, and release it to theaters, where it actually performed fairly well (it made about 20% of the profits The Blob made, which, while not good compared to The Blob, is still what Harris himself calls "pretty good") and was apparently popular with kids at the time who liked the monsters in it.
3. Two separate cuts exist, the Harris-produced one and the original one made by the college students, which adds to its value as a curiosity if nothing else.
4. The effects were done by Dennis Muren who went on to become a big time effects guy in Hollywood, so the film has some historical significance on top of all of this.
So it's a relatively obscure monster movie begun as a lark by some students that was then taken by a well-known producer and turned into an actual movie that (if the audio commentaries are to be believed) performed well at theaters and got good word of mouth. And it has since become a cult favorite among special effects lovers and monster movie buffs. So Criterion released it for people who were interested in special effects and film history.
On its own, Equinox is just a curiosity, but by releasing it, Criterion can include a lot of special features about its history and its place in history, specifically how producers sometimes look for student films to pick up for distribution, to say nothing of Muren's involvement and how it got him started. Whether it's bad or not is irrelevant. Equinox was considered culturally or historically significant for these reasons, so Criterion released it.
I mean, really, how many times will you look under Jabba's manboobs?