The event of Mubahala took place on 24th Dhu'l hijja 9 AH and the event of Ghadir Khumm took place almost a year later on 18th Dhu'l hijja 10 AH.

The Prophet (s) passed away within a few months of Ghadir Khumm. At the first anniversary of that event Abu Bakr was in power and unlikely to celebrate an event that would undermine his own rule.