It's a really, really bad movie. But the director's choices are captivating in how bizarre and wrongheaded they are.
If you like this kind of thing (and not everybody does), it's natural to want to have that be a social experience. Partly because it's always more fun to laugh in a group, but also to sort of look at each other and say, "Did something that bad actually happen on screen?"
MST really clicked for me and a lot of people, because Joel/Mike and the bots on screen clued me in to this whole way of watching movies that I'd otherwise have only been able to pick up on in a live theater. And, realistically, it was the only way these gems of entertainingly bad cinema were going to get airtime: without the jokes, many would've been much too boring otherwise.
Another good example of this is Mystery Science Theater 3000. They've had the reaction video patented and perfected since the 1990s, and it still feels like nobody can quite hold a candle to how well-presented MST3K is. They also have a great (official!) YouTube channel[0] that uploads their best episodes. Go check it out if you've never heard of them, or just want to come back for the laughs.
[0] https://www.youtube.com/user/mst3kofficial