u/Poka-yoke1

TLDR: Unavailability of indirect fire weapons

Modern warfare relies mostly on indirect fire weapons: artillery, mortars, missiles, etc. They do not travel in a straight line. A marksman has to adjust his sights due to the effect of gravity on a bullet. Direct energy weapons on the other hand need a direct line of sight to hit their intended target. In a universe where only these weapons are used, you could see elevating the firing platform to overcome the curvature of the planet or other obstacles as the only viable solution. Given that the Empire was reliant on air superiority, for the most part this wasn't a problem. In the limited case where there were no spaceships available or say, jungle canopy obscured reconnaissance, the ATAT/ATST would be deployed. None of this solves why they were deployed on the battle of Hoth. It was a glacial planet with a star destroyer orbiting overhead. Even if there were some atmospheric interference that made their blasters less efficient, the main cannon of the ATAT would be nowhere near as powerful as the main gun of a star destroyer.