Probably because it was gratuitous and just there so people would go, "Ooh, Chewbacca." There are also people who say it ruins continuity because Chewbacca would believe in the Force having worked with Yoda and maybe objected when Han was making fun of Ben for it. Hell, he even makes fun of Ben himself when they're in the Death Star. If we can accept what Peter Mayhew really said in the mask as canon, then his growling there mean, "That old man is daft!"

Plus, I saw that movie twice, and both times, I just totally stopped paying attention at the Wookiee battle. I didn't mean to, but it just wasn't interesting enough to hold my attention.