In the Dune universe, when a laser weapons hits a shield, both are destroyed in a nuclear explosion reaction.
So instead of building nuclear weapons, wouldn’t it be easier to tie a timer and a “parachute” to a laser gun and drop it from orbit onto your enemy’s city?
Well in addition to the other answers, out of universe, the answer is power of plot. Simply nuking the shit out of everything wouldn’t really make for an interesting story.