Its not necessarily the similarity between the 2 creatures....its just too much monster for one film, let alone to all be jammed into the first 3rd of it.....
The Rancor is a monster Luke must battle, part of the character's voyage or story arc....the Sarlaac is a mysterious hole in the ground, representing a fate worse than death....it is symbolic more that an overt threat that must be fought against physically........we shouldn't see it overmuch, apart from its mouth....its the Boba Fett Effect....explain too much of it and it loses its cool.