At the time, animatronic technology wasn't thought to be advanced enough to pull off Yoda.
So, in a solution that would make Karl Pilkington proud, they decided to try putting a trained monkey in a Yoda costume, including a full Yoda face mask. Rinzler showed a picture of the monkey on set, but he explained this idea was quickly abandoned when one of the people who worked on the primate scenes in 2001: A Space Odyssey pointed out "Look, the monkey's just going to pull off the mask over and over again. It's never going to work."
You would have thought one wouldn't need first-hand experience to know that, but the creative team decided to junk the idea and enlist Muppet mastermind Jim Henson to create the world's first animatronic muppet. To show the extent of the character's Muppet origins, Rinzler showed a photo of Yoda hanging out with Kermit and Miss Piggy in the swamps of Dagobah.
Link | Image: Lucasfilm