What happened to them. Well, they're a pain in the ass to maintain. Chemicals in the big bag to keep the water from growing bacteria/algae. Heat needs to be kept on regularly or they are too cold, so if the power ever goes off, you can't sleep. They're a total pain in the ass to move, requiring the whole multiple-hour-long procedure of draining the entire bag out, then relocating the bed frame, then re-filling. So plan an entire day if you ever want to move furniture around.

And while the vast majority of them never sprung leaks, all it takes is one to do it and flood out a room for landlords to freak out and write "no waterbeds" into their leases forever.

They also could be very weird to sleep on long term. As in, you had to locate yourself and your partner exactly in the same spot, or you'd roll either in or away. Supposedly in later years they fixed that with more support inside the bag, so you weren't just sleeping on a big wad of water sloshing around every time someone moved.

They were great for one night, or a weekend. They were great as a novelty. As a long term functional thing they definitely had issues. And then landlords said the hell with it go play someplace else.