Boba Fett was actually first introduced in the notoriously horrible Star Wars Holiday Special, as an animated character. In the gap between Star Wars and Empire Strikes Back, he developed a fervent following for reasons unclear to me. And then he did manage to capture Han Solo, so I suspect his role as adversary got a lot of attention.

Plus, the armor. The armor is just plain cool. Once upon a time, I played in a Star Wars LARP and some guys had made Boba Fett-type armor. They got respect from everyone around them.