Because it requires specific orders, and Sidious didn’t give that order to the Clone Troopers assigned to Anakin.

See the top answer here: https://scifi.stackexchange.com/questions/114261/how-did-anakin-skywalker-survive-order-sixty-six

I’ve never understood why the enemies of Vader never created an army with battle armor based on the style Vader wore. It would confuse the stormtroopers, wouldn’t it?