Jellyfish are very delicate and extremely hard to keep in tanks for that reason. They are mostly water with very thin membranes as their body structure. Any sharp object can rip them apart fairly easily. Of course, there are lots of larger jellies that have evolved to be more durable, or can grow big enough to withstand damage (like the Stygiomedusa gigantea, or the larger fried egg jellyfish specimens). But the vast majority are small and mostly helpless, except for their unique stinging cells, but that doesn’t affect their ability to survive in a tank.
In their natural environment, they usually just float freely with the currents in the open ocean, so there’s nothing for them to bump up against (except predators). They aren’t strong swimmers either, so they can get stranded or trapped easily if the current washes them onto the shore or pushes them into a confined area.
In order to most closely replicate their natural environment, jellies need tanks with rounded edges to keep from getting stuck in corners, a gentle current to propel them around the tank, and a small amount of food suspended in the water. They have very specific requirements for temperature and water pH too. No gravel since they could scrape against the grains and tear up their membranes. No plants since they could get stuck in the leaves.