because they hadnt gotten to draenor yet?
just like how humans havent gotten to mars yet. asking why there arent old gods on draenor just because the titans were there is like asking why if there was likely life on mars why humans arent on mars.
the answer is simple. the universe is vast, its large. the old gods had only just started corrupting draenor by whispering too it. theres no reason to assume that had we not stopped the arrakoa in bc or stopped them before they got started in wod that the old gods wouldnt be having a party over there by now.
also really? you need a source for azeroth having blown up multiple times? there was an entire trilogy about it and an entire expansion about the 2nd time it happened