Why does the Bible say that you shouldn't eat hares?

Because they chew the cud, but don't divide the hoof.

These ye shall not eat of them that chew the cud, or of them that divide the cloven hoof; as the camel, and the hare, and the coney: for they chew the cud, but divide not the hoof; therefore they are unclean unto you. Deuteronomy 14:7