Why, if the kingship is in Yehudah, did Shmuel pick Shaul who was from Binyamin?

The Ramban explains that in the source itself that the kingdom belongs to Judah, the wording is "The (ruler's) staff shall not leave (!) Judah" (Breishit 49, 10), that once kingship comes to Yehudah it should not be taken away. On the other hand, Shaul was appointed by God Himself (through his prophet Shmuel, who was directed to anoint him), despite coming from the tribe of Benjamin, because the kingship is not "leaving" or being "taken away" from Judah, for David had not yet received it.