On account of their unbelief, whereby they doubted and distrusted God's power to destroy those who had chariots of iron and so gave way to their own fear and sloth, whereby God was provoked to withdraw his helping hand from them, and so they were really made impotent, as they were unwilling. See Joshua 17:16.
Related Commentaries of Judges 1:19
Judges 1:19
19 And the LORD was with Judah; and he drave out the inhabitants of the mountain; but could not drive out the inhabitants of the valley, because they had chariots of iron.