@lain
possibly because pants got more affordable and materials and patterns for pants got good enough to allow for sufficient flexibility
the main upside of a skirt is its simpler design and manufacture and maybe that it doesnt limit your range of motion ass much.(you might reveal your balls tho if you actually use this feature)
with both of those gone or at least largely mitigated there is little reason to not just wear pants and geet all the benifits that they bring.