Do you remember that movie Weekend at Bernie's? It's about two guys who go spend a weekend at their bosses, but he dies so they have to pretend like he's alive so they don't look guilty. That's what might have happened here with C3PO. The actor died of heat exhaustion inside so they just left him in there and had to pretend like everything was fine so the movie wouldn't shut down. "Does he need water?" / "No. He's OK." / "Why are there flies coming out of his breathing hole?"
We already saw from a previous slide that C3PO needed a standing gurney to get rest, so I can see why he would need help walking. Imagine how much the corners of the suit are pinching him. He had to worry about that every time he moved his arms. I get chills just thinking about it.