If I were designing armor or apparel intended for actual combat, I would not include big handles all over it to snag on things or allow someone or thing to easily grab hold of the wearer. Same reason that UFC fighters don't have long hair.
Smoother is better if you don't want to get hung up on some object at the exact wrong moment and lose the most valuable of all combat assets: Mobility.
Also, you don't build armor with flat and definitely not concave parts in the vicinity of high risk areas of the body. This is why breastplates with boobs on them (a staple of recent Bethesda titles) are nonsensical and would be useless because they would be collecting a blow toward essential parts of the body instead of deflecting it away.
The t51-b is primarily constructed of convex curves that would deflect a projectile or cause it to glance off of the wearer and away from their delicate parts.
Notice that even the visor is smoothly curved around the face and there is not a depression around the eyes that would collect a ricochet toward the relatively unprotected eyes.
This is good design, whereas the t-45 and t-60 would take the same bullet fired at your face and collect it toward your eye socket.