Why are some western stirrups slanted?

They're called roper stirrups and they slope or slant to the outside. The idea is that this provides a more natural and comfortable position for the foot. The stirrup is flatter beneath the foot - flat western stirrups often don't support the entire foot. They're preferred by ropers and some long distance trail riders.

You don't see slanted English stirrups because the lighter stirrup supported by a thinner strap will shift to fit better under the rider's foot.