Presumably because tanks are hard to hide, not because they don't want to. Guerrilla warfare warfare is not a middle eastern thing; a strategic manual we were encouraged to read in the Norwegian army told us we should source food and supplies from the local population and fill sugar on the gas tanks of the enemy. Assuming you've lost tanks and airspace first, of course.

From what I've heard IS has all kinds of heavy weaponry and hold territory. Seems like hiding would be hard.