Now Joshua was old and advanced in years, and YHWH said to him, "You are old and advanced in years, and there remains yet very much land to possess. This is the land that yet remains: all the regions of the Philistines, and all those of the Geshurites..."
Joshua 13:1-2

Joshua wasn't going to live long enough to complete the conquest of Canaan. Allocating the land to the tribes of Israel was a higher priority than completing the conquest, and it had to be done before Joshua died or else there could be a civil war.

The KJV mistranslates v1 to say "Joshua was old and stricken in years". It implies that his age had affected him negatively, but the Hebrew doesn't say anything like "stricken". It says "bo b'yomim" or "entered into days".