The idea behind powered armour is that because it's self-supporting and augments the wearer's strength, it can be a heavier (not just in terms of armour, but also life support, electronics, flight packs, weapons and all other systems) without encumbering the wearer. This is true even if gravity manipulation compensates for the armour's weight, since mass and recoil can still be cumbersome or even dangerous without increased strength (though if inertia control is also available, that might be able to compensate.)
A 'powered armour breastplate' with no suit would just be a very, very, very heavy breastplate, not practical for combat in space or on the ground. While it's not physically impossible for someone to wear 300 pounds of unpowered armour, it's almost always a very bad idea, and so rarely a good idea that no one is likely to bother creating it, or using it more than once if they do.
If high-powered shields are dangerous to someone not wearing an environment suit, not only too massive to use without strength augmentation, that'd be another reason to avoid powered armour without the suit.