Why would they be?

> Is it just not cool to do anymore or something?

Mostly. It's not like you can give someone a super-death penalty, and when progressive criminalization has made it so that everything that qualifies as treason is also covered by other crimes that carry similar sentences but don't require two witnesses to the same over act or a confession on top of the standard criminal “beyond a reasonable doubt” burden of proof, why would a prosecutor ever bother with treason charges except as a political stunt?