Why did he want to renew the kingship?
Rashi: Before, not everyone agreed to it. Now, all agreed to it.
Why did he want to renew the kingship in Gilgal?
Radak: This is in honor of the Aron and Ohel Mo'ed which were there initially when Yisrael entered the land, therefore they honored the place, even though now [the Mishkan] was in Nov.