Because different states have different regs. In Minnesota you can trap and hunt them year round.

Most trapping regulations are centered around selling the catch so are set around when fur is prime.

It is only recently that some species moved into the east, coyote, or grew in numbers to become a nuisance to landowners, beaver, and seasons were liberalized to allow year round take. Then depending on state and local tradition those may be legal hunting or legal trapping or both

For instance, Georgia just allowed year round raccoon when less twenty years ago you couldn't even trap raccoon in the northern counties.