On some breeds, such as the Badger Face, the underside of the tail is woolless and the wool is coarse and long and hangs down the sides and back of the tail, so not near the anus. Long tails also offer some protection to the udder. On Down breeds and so on, with finer fleeces, the wool is shorter and fluffy and will almost certainly get in the way when the sheep defacates, partcularly when the fleece is long anyway in late WInter and early Spring. The wool surrounds the udder anyway, so a long tail isn't necessary for protection.