That may have something to do with the non-EU player limit rule in La Liga. At the moment, here are Barcelona's non-EU players: Arthur, Vidal, Suarez, Neto, and Ronald Araujo. Moussa Wagué's another, but he's currently on loan. When Wagué comes back, Barcelona will have to drop one player from their team.
The Arthur exit rumours tell me that Barcelona may still value Suarez and Vidal. They probably don't want to go after another backup keeper (don't know why they exchanged Cillesen in the first place), and they do appear to like both Araujo and Wagué. Since Barcelona are only allowed 3 non-EU players in their matchday squads, Arthur may not be able to make it ahead of Suarez, Vidal, and Neto. If that's the case, then they're probably trying to cut their losses and exchange him for an EU player of the same profile.
This has nothing to do with his ability; Arthur's a great player. However, he's probably being seen as a luxury rather than a necessity. If they really valued him that much, they'd be looking to get him Spanish citizenship.
EDIT: I forgot about Coutinho. Barcelona really did screw up their squad building & management.