I guess he wants to allow his party to find a new leader. Also, he gets to hold onto power for a couple more months by not leaving immediately.

Also, the leader of the NDP party, Jagmeet Singh, gets his pension on Feb. 25th. This is relevant because Trudeau's Liberal government is a minority government and requires a NDP alliance to prevent an election. Singh doesn't want to risk losing his House Representative status and thus risk losing his pension, so will always instruct his party to vote to keep the Liberal party in power until his pension is secured. Nothing will happen before Signh secures his $2.3 million CAD pension.