I don't think we can say that Kert was a "big star" at that point. WSS was a ways back by then, and I think his only other major claim to fame before Company was playing Cliff in Cabaret toward the end of its run.
That, and I don't think everyone puts "star status" ahead of getting work. Besides, it was a chance to work with Sondheim again, and a lot of other major talents. And he was the standby to the role that would carry the show, were he to go on. What's to turn down?