True, the actor could have been wearing a blue shirt or whatever, but it's easier for the actors to keep in character if they are dressed in character. It's a bit off putting if Harrison was acting next to a guy in jeans and trainers...get my point?!?!
Also, the chess game was probably the reason they did'nt bother with Jabba. It just wouldn't have worked.
My other point. The Jabba in the Marvel comics is actually a character we see in the background in Mos eisley. You have to look closley but he is there walking around. I'll have to watch the film again to tell you where exactly, but belive me, he's there!