Because the game isnt stuck in a vaccuum. Its an ever expanding, evolving world.
Over time, the races and cultures are becoming smarter, more organised, more civilised.
It's been over a decade of our time, over 3 decades ingame. Every race has evolved to suit the enviroment around them, which is one that requires tollerance and cooperation with allies, skeptism of non-allies and cautius optimism of peace from enemies.
Orcs could not prosper with war. Thrall understood that while war may be neccesary/inevitable at times, peace was ultimatly required for the Orcs to grow as a people. Other races have figured this out on their own as well over time.