The Death star explosion not affecting Endor was more believable than a bunch of primitive Ewoks taking out an advanced force with overwhelming technical superiority.
I suppose Lucas wanted to sell more Ewok teddies than At-At toys.
The whole Ewok thing was ridiculous, and I was a bit annoyed and puzzled by it as a 9-year old in the theater. Lucas was getting weird and punch-drunk by the end of the trilogy, anyhow, and the Ewoks were a symptom of it. He needed a real director to reign in his excesses (like he had in ESB with Irvin Kershner).
BTW, I recall reading that the Ewoks were supposed to represent the Viet Cong defeating those “oppressive American capitalist-fascist pigs.” Who knew bloodthirsty Commie totalitarian devils were so cute and loveable ?