They knew what Tito did to the Nazis, and the Chetniks.
Well, They took care about other ruthless dictators. And they had like 10:1 military advantage. They even had some friends in Yugoslavia (see one of the previous posts)
They could run him over pretty easily!
I always felt that they sort of needed an enemy, so they could shoot people in their countries, and blame them on Tito? Kind of having Tito as a straw man. But, I do not know.