Maybe they did it because the original Rancor is boring. It was scary for the first time you saw it in ep6, but then............even JK3 mutated a Rancor
They made it because the "Bull Rancor" is Tougher/Stronger/Scarier than the original.
Bink's Sake
"I've barely eaten anything for many decades now. It feels like my stomach shriveled into my back........though i don't have a stomach or back because i'm a skeleton! Yohohohoho! SKULL JOKE!"
"My eyes have never seen such a pretty woman! But i'm a skeleton so my eyes don't see much."