Hmmm. Interresting question.

I think the fact that we wear pants today could be a result of modern or near past cultural effects (after all if I remember right then Herodotus mentions that the Skythians and/or Persians wore pants, but views this as a sign of barbarism - even some EB units descriptions mention that Hellenes looked at pants as a clothing of Barbarians).

However it could have had practical reasosn aswell: many nomadic peoples (who had an extensive horse riding tradition - so most of them rode horses al day) seem to have used pants as a constant part of their clothing because it is more confoteable and easier to ride in and protects the legs, the skin and other important parts better then any clothing similar to a toga or a kithon could (they seem to have used underwear similar to what we have today for the same reasons). Plus some of these peoples lived in colder regions then say Numidians, Romani, Arabs and Hellenes so it also helped to defend against the cold. (we also know that later on Romans also wore pants on the northern frontier to keep there legs warm)

Most not Mediterranean European peoles seem to have preferted pants out of similar reasons.

Why it became dominant later on I can't say for sure.

(Interresting fact however: I heard that Jeans were originally meant for miners - because these where very resistant and though clothes which would not get used of so fast (the miners often neded to slide down shafts which were too step or too smal to pass normally - no better way to ruin your clothes).