Originally Posted by Vanishing Point
Probably because they don't have bears. In bear country you are encouraged to hang your edible smelling gear from trees.
Also, ropes can damage the tree bark and nitwits are always pounding in nails and leaving them there.
Sometimes, if you explain to a rational park ranger how the hammock hangs with straps that don't harm the trees rather than ropes, they will let you skate on that.