The reason land hermit crabs (yes they live in groups but I got the joke about them being hermits) die in captivity is the same reason fish die. Anyone want to guess? Go on, have a guess.
Dirty conditions.
People keep them in a small tank with sand or gravel, they have a container of food and water. The crabs eat and poop in the container. Bacteria and fungus start to grow on the rotting stuff. The crabs get infected and die.
We used to tell customers to wash their hermit crab tanks out each week to keep them clean. We had customers that kept their hermit crabs alive for years.
You just need to let the crabs out each day for some exercise. A tiled floor is fine, just make sure nobody comes in and steps on them.
Clean the food and water bowls each day. Replace the food and water each day. Feed them a varied diet consisting mainly of fish foods, and fruit & vegetables. They should have a container of fresh water (free of chlorine/ chloramine), and a container of saltwater.
Clean the tank and substrate each week. Just get someone to babysit the crabs while you take the tank outside and hose it out. Wash the gravel/ sand in the tank a few times. Drain the water out and set it back up. Once a month you can bleach the substrate and sand if you like, just rinse well and let it dry before adding the crabs. If the tank is washed each week bleach should not be necessary.
Let them get some sunlight each day but don't have the tank sitting outside in the sun because you can cook them if it's really hot. You can take them outside and let them run around a clean sandpit for an hour or so during the day. Some people use a plastic tub (clam shell, kiddy wading pool) with a couple of inches of beach sand in and have that for the crabs to get some exercise outside.
Get small hermit crabs and handle them each day. Bigger crabs tend to bite and some small crabs will too. Try to hold the crabs at the pet shop before you get them. If they bite, then try another one.
Don't let them get too cold. Land hermit crabs are from tropical regions and die if it gets too cold.