Death stars are equipped with a massive single gauss cannon capable of destroying just about anything with a single shot, be it destroyers or moons. In order to provide the energy for this weapon, huge parts of the death stars innards are used up for power generators. The size of the death star also limited its travelling speed, which is very low. It is being told the captain often helps pushing it to increase speed.
Only huge and advanced empires have the manpower and extensive knowledge required to contruct such spacecrafts with the size of a moon.