Well, aside from the fact that there is only the one set in the game (unless you are counting the Winterized set from Operation: Anchorage), the T51b armor has no repair list entry in GECK. That's the main reason it cannot be repaired by the player.
I suggest going to Canterbury Commons and talking to Uncle Roe about investing in the merchants. If you invest 700 caps total on Crazy Wolfgang, he can repair your T51b armor better than any other merchant in the game. Investing in him completely will raise his repair skill to 88 (I think). He wont be able to repair the armor 100%, but even at 88% it still has a better Damage Resistance than any other power armor.
If you have Broken Steel, you may be better off wearing Enclave Hellfire Armor. It has the same DR as the T51b, but it also has added fire resistance and a strength bonus and you can repair it yourself using other sets of Hellfire armor. Plus it looks really sinister

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