1)
Why did they not dispossess them?
1.
Rashi: They transgressed; Hashem commanded "v'Horashtem" (Bamidbar 33:52).
2)
Here it says that they did not dispossess them. In Yehoshua (17:12), it says they could not dispossess them!
1.
Malbim: In the days of Yehoshua, they were unable. Now they were able, but did not want to.
