i think it's largely because jail is seen as not being punishing enough.
and it isn't. your stay in jail isn't anyway. what is punishing about it -- and too much -- is the time it takes from you. it's the job, the house, the dog, the wife, the life you lost because of it. the stay itself isn't punishing though.
a lot of people feel that criminals should be punished in very real, very immediate terms, for violating the rights of others. jail is "soft" because it doesn't do that, instead it shelters and feeds them.
i think punishment should escalate to the point of jail, but before jail happens, things like whippings should be used. jail instead screws up people's lives instead of teaching them lessons through pain. only people beyond redemption should go in those jails, and at that point it's not about punishment really but about removing them from society.