Well the 501st legion was Fett clone pure legion led by Darth Vader during the Galactic Empire era. And many of the Storm Troopers right when the Republic was formed into the Empire were Fett clones. Also most storm troopers were flash trained and were from many different genetic templates, and they preferred to just storm in and shoot aimlessly compared to them. They were cheaper to make and maintain so the Empire began to use them along with some humans and some aliens. And the armor didn’t help much ether. Thats why most storm troopers were shitty shots.