Why was this character uncredited?

Cameo roles are often uncredited in films. Usually it's to help keep the appearance secret, so the actor's name doesn't appear in a published cast list. In some cases, an actor will agree to appear in a film with the express agreement that their appearance will not be used in any publicity for the film. See Brad Pitt's (very) brief) appearance in Deadpool 2.

Sir Alec Guiness appeared in a 1995 thriller called Mute Witness in a very odd way. The director had an opportunity to meet the actor in Germany, before he'd even begun production. He quickly pitched a scene to Sir Alec, and he agreed to film it, if his name was not used for publicity. He is listed in the credits as "Mystery Guest Star".

News that Harrison Ford was to return in the film would have been a massive spoiler, so secrecy was surely the watchword.