My 2 cents: I could easily be wrong, but probably for the same reason he did not call for the Hosts of Cherubim to fight against his enemies, I believe.
There's the Lamb, the 1st coming, then there's the Lion, the 2nd coming.
Abraham didn't get rid of the Canaanites either, though the land was given him; it was reserved for the later Joshua (who, too, didn't really get rid of them all - nobody did. If anyone finally got rid of them, it would've been the Romans, Arabs or Mamluks.)
Why did Yeshua not rid the land of Canaanites?
--because Yeshua must fulfill other prophecies first, then the triumphant ones in its ripened time.
--this question is typical from my anti-missionary brethren. the obvious defect of their argument is that they wanted G-d and his Messiah to act ACCORDING to their personal expectations. reminder: no man is bound to live according to your expectation, nor G-d. be wise to observe the "signs" of times, not the imaginations of other men. in their appraisal of messianic prophecies they become too dull that the ancient pharisees were much better in their mastery of the passages than the bland attitude of today's leaders. slach liy.