That same level range there are several other good options, but the reason that parties go to the tree is because it's the best exp for most parties.
Dire Bats in KRT don't have that great of a camp for fast chaining, the pulls can be long, and you'll have to toss in a scorp every once in a while.
Raptors and Spiders in kuftal are nice, but spiders link and raptors hit hard and fast, although they die fast. They're also a level or two above tree crabs.
Stygian pugils in Tor. Canal are nice, but it's a hassle to get to them and most parties are too afraid to exp in the zone because they don't know it.
Sand lizards in kuftal are nice, and only a level below tree robbers, but good luck with them if there's people camping amemet, which the gilsellers always are. The crabs in kuftal are 2 levels down from the tree ones.
Goblins and crabs in gustav are a couple levels above tree crabs, but they hit a whole lot harder, and diaga can really mess up a party without erase, and you won't have it from /whm at that level.
Bunnies and lizards in cape terriggan are nice too, but you need to watch out for gobs in the area which are too difficult for parties at this level to exp on.
Crabs in cape are a couple levels above tree crabs, and the pugils are a couple levels above them, and you'll probably have to fight pugils just because they are in the way and aggro.
There are plenty of other good camps, but when you're 53-56 the best spot for your party is boyahda crabs for almost all party setups. They don't hit hard, they don't have any devistating TP moves (provided you have a dispel) and they are weak to ice and lightning, 2 of the easiest elements to make a MB for. Toss in the fact that there are 2 camps for them, with 9 spawns each that are all in a very small area close to camp, and you can easily see why parties exp there.
I don't see why NPC levelers can't recognize that there's much better spots for solo exp. Camp the 5 hectaeyes in KRT and kill dire bats waiting on respawn. If your NPC is a healer they will take care of paralyna, if they are a tanker sub whm to paralyna them. You'll get faster kills than crabs, and maybe get a burning eye or two, which sell for what now, 200k each?