I really don't know why, to be honest. I mean one of the merits of a true brass hero like Cap is that he doesn't stop, and he always comes back despite what agony/torment he's been through, unless he's just outright physically unable to do so.
With Bucky there I see one of three things happening:
1. Bru keeps going with Bucky as Cap, until he leaves the series, and let's his successor take care of what to do next.
2. They keep Bucky in the suit pretty much to show that he's not really Cap, have him fail in some way, and then Steve comes roaring back to beat down whatever as the one true, blue Cap.
3. Bru just skips the fat, gives the shield back to Steve, and puts Bucky elsewhere