Because, while Terminator armor grant heavy protection and heavy weaponry, it also make the wearer super slow. Sure you can teleport but it also mean you're going through the Warp.
I still wonder why hero marines dont wear that thing and stick to their standard armor whilst carrying unique weapons.
Well, when it comes to heroes, then for example Crowe has the artificier armor, which looks like standard GK power armor, but gives him a 2+ save anyway.