We can find it on the Dawn of War III units description:
"In order to move so quickly around the battlefield, Assault Marines are equipped with light armour. As such, you can effectively counter Assault Marines by saving your piercing shots for when they strike."
Assault Marines are wearing superior mk7 heavy power armors using the most advanced technology known by the Imperium of Man (do not interpret as forgotten technology please) just like all their brothers.
You think that Relic really want to make "line troops" out of Astartes?