1)
Which Kena'ani is discussed?
1.
Rashi: It is places that were received in the lottery, but were not yet conquered.
2)
Why did they ask only after Yehoshua died?
1.
Malbim #1: In his lifetime, everything was from Hashem's word; they did not need to ask the Urim v'Tumim (Eruvin 63a).
2.
Malbim #2: In Yehoshua's lifetime, he headed the army. Now they asked who will go first.
3.
Malbim #3: In Yehoshua's lifetime, all conquered together. After he died, each Shevet conquered its portion. 1
1
Malbim: It was important who will go first, for if the first will not succeed, the Kena'ani will say that after Yehoshua died, Yisrael's strength departed.