Unless you're a total geek or a die-hard Mark Hamill fan, one of two things just happened: either you thought, "Mark Hamill was in another movie?" or you tried to dredge up some distant memory of something else you'd seen once long ago--probably Corvette Summer.
In fact...tell the truth. When you hear "Mark Hamill," you picture this, don't you?
Mark Hamill is a crappy actor. I have no idea whether or not this is true at this point, but it's one possible explanation.
A couple of bad choices right out of Star Wars set him on the wrong path, and he was never able to regain momentum.
It had something to do with the accident he had near the end of filming the original Star Wars movie. Lore surrounding this accident is dramatic and varied, but at the time it seemed generally accepted that Hamill's appearance had been affected.
He's perfectly happy doing voice work and stage acting and hasn't tried to follow up his Star Wars success with other big movies.