Just to make them more unique, and to give a reason to use the earlier game suits besides looks. For example, in the last two games, the T-45d had the highest strength bonus of all power armors. So it's unique upgrade could be another boost to melee damage, giving it the highest melee damage bonus of all the armors. And considering it's also made of the heaviest material, the upgrade could give a higher stagger resist.
The T-51b could have it's chemical hardening upgrade from Fallout 1 and 2, giving it a moderate resistance bonus to all damage types, as well as a large increase in item hp. This would give it the most rounded defenses, while also making it the hardest to break.
The X-01, which is essential Enclave advanced power armor, it could have increased movement speed, as well as lower Action Point drain. Being made of lightweight metal alloys, reinforced with ceramic, it's hinted to be the least heavy/cumbersome of the power armors in previous games.
As for the T-60 or Raider variants, I'm not sure. They're new, and not a whole lot to go on there lore wise. What do you think?