Kasdan co-wrote ESB, ROTJ, TFA and SOLO along with his son so he's written a sizeable chunk of the 11 theatrical films. Granted/however, I think part of the reason he was brought into TFA and SOLO is probably because they were fairly Han-centric movies-and of course, ESB is perhaps Han's main showcase, showing some character growth as well as Han's instincts helping to keep the Falcon hidden from the Imperial fleet (Apart from the hyperdrive issue). It also doesn't hurt that he co-created Indiana Jones (Which I think got him the Star Wars gig in the first place, although EMPIRE came out first), although apart from a few surface similarities Han and Indy are quite different. (Indy for instance is a terrible pilot).