The bottom opening of the gaiter is not wide enough. It wont sit low enough on your shoe to fasten the strap around the bottom of your boot.
I'm not sure. So, I don't want to give you an incorrect answer. I think it may have to do with the fact that it connects to the laces on a boot. With a bigger boot you'd want to make sure it opens enough at the bottom to connect to the front while covering the back, near the heel.