I opine it is because they served no useful purpose outside fashion conciousness, and are a pain in the neck. Kilted, they aren't going to keep you any warmer, and they aren't going to keep bugs from crawling up your legs.
I could be wrong, but I would think spats were originally intended to keep the hose from getting wet or snagged when walking through the countryside, since roads were not very well developed in the Highlands back then. Nothing more than a utilitarian function.

As roads and transportation became more modern, the use of spats went away. Some people clung to them out of a sense of tradition, but that eventually waned.

I personally love the look, and don't mind the hassle of lacing up gaiters or leggings. I wish they would come back into style.