I guess because you can still make a prequel movie set in the pre-Rebellion days of Han without involving a reference from Empire and Han meeting the Falcon for the first time. In other words, something like the old Han Solo Adventures where Han already had the Falcon, but before he met Leia and Luke.
It would still be a prequel, and probably “unnecessary”, but at least it would just be a fun movie, like the old spirit of the franchise in the pre-oEU days.