I guess you're right, I'd never thought of it that way. In the first two movies Han is integral to the plot, transporting everyone and keeping them safe, right in the thick of it. In ROTJ, he's overwhelmed by events. Leia is still big politically, but especially as being a potential Jedi and a sister she gets caught up in Luke's plot. Luke as the title character of course plays a big role. Han gets dwarfed in the struggle; though, without an important role to play. We follow him because we need to know about him, not because he is doing something we need to know about.

Interesting. Poor Han. At least he gets the girl.