Why did Yakov ask Yosef to swear?

Part III (and final) of Rabbi Shmuel Posner’s class on Parshas Vayechi.

Why did Yakov have to ask his son Yosef to swear that he would burry him in Eretz Yisroel? Yosef’s promise was not enough? Why didn’t Yakov want to be buried in Egypt anyway?