That's because armoured boots use the ankle and foot slot, whereas shoes only use the foot slot. Beast races still have the ankle slot open to them, so leggings work.
Okay, I just tried using Adul's Legging's mod on a pair of exquisite shoes, and nothing happened. I tried to make sure it worked by tossing a pair of armored boots in there, and it worked. I also read the readme, and it says nothing about being able to convert nonarmor shoes, like common shoes, into leggings.