The bracers require attunement. The armor doesn't. That's the crucial difference. You only get three attunement slots, so they're a valuable resource.
Though I have to admit that it's hard to justify the arrow-catching shield being on the same level as bracers of defense.
+3 BoDs require attunement, we can see that, but we don't know whether +3 armour also does, AFAIK. Even in the late playtest we only ever saw +1/2 armour.
I dunno about anyone else, but I'd be pretty surprised if there was any +3 armour that didn't require attunement, personally. Tell me if I've missed some that didn't.
That said, I have literally no idea why people think these things are somehow overpowered, and this thread has done zero to clarify that. It seems to just be a sort of assumption that we are all expected to take for granted. As has been illustrated, they are, at most, the equivalent of +3 armour, but usable by Barbarians/Monks/Casters. It's unlikely, in actual/practical play (rather than pure theorycraft) that any of those people is going to be beating a Plate-wearer on AC, let alone Plate, Shield and possibly Fighting Style stacked together, unless said Plate-wearer doesn't have magical armour.
The only problem I see is that the Arrow-Catching Shield, as you seem to be saying, also requires attunement and is "rare", despite being wildly less powerful. The extremely short range on the ability combined 1/round nature of it means that it's extremely corner-case in it's usage (if it had a 15' or 30' range it'd be a different story). Presumably it's one of those slightly-shoddy low-level items that one eventually moves past.
As for the reward vs. expectation debate, it's seems like much hot air either way. If you give out magic items as "rewards", you are creating a particular expectation, that being that if a PC does something that you, the DM, think is cool/hard/otherwise pleasing, he may be rewarded with a magic item suitable to him. Obviously if it was useless, it wouldn't be a reward, so there's that.
Similarly, DMs who do use wish lists and the like actually end up playing a very similar way in actual play - they're not handing out items "at request", or the like, they just ensure the bulk of the items that they do let the PCs get their hands on are ones that the players actually want.
In the end, I think it's fair to say that in the very vast majority of D&D campaigns of any edition, DMs place items with an eye to what the PCs in their game will actually get either some use out of, or a kick out of, but they also place ones which merely seem appropriate to the situation/place/enemy, or which they personally think might be interesting to introduce, and may well introduce a few random ones too. There probably are campaigns where item placement is exclusively random-by-table (at one end) or exclusively player-list-driven, but I think it's fair to say that both are pretty rare approaches. So the whole reward vs expectation thing strikes me, personally, as posturing. In reality 90%+ of DMs are both rewarding and to some extent playing to player expectations, even if those expectations are ones that they've shaped.