Malbim: He caused that Yehoshua not conquer all of them, to test if the generation after him would dispossess them and not make a Bris with them. After Yehoshua died they did not obey, so Hashem decreed not to dispossess them, and Bnei Yisrael could not overpower them, like Yehoshua said "Im Shov Tashuvu... Lo Yosif Hashem Elokeichem Lehorish..." (Yehoshua 23:12,13).