Why could the men of Judah not drive out the people living in the plains?

Answer: They had chariots of iron

The LORD was with Judah and they took the hill country, but the people of the plains had chariots of iron.

“The LORD was with Judah, and they took possession of the hill country; but they could not drive out the inhabitants of the plains because they had chariots of iron.”— Judges 1:19 (BSB)