Profitability aside, GCU couldn't have been popular with publishers. While we all know games go on sale pretty soon after they release, publishers are still trying to push the idea that their product is worth $60, and you should pay that $60 right away. If you've got Amazon and BB essentially saying "no game will be $60 for you, they're all at least 48!" then customers are going to have a lower image of the "value" of your product.
Heck, just look what happened when the Epic Games Store took $5 off of all of their games earlier this year (and EGS ate the cost of that discount). Even games they had exclusive rights for, like Borderlands 3, didn't want to be part of that sale, because they didn't want their perceived value to change.
And remember what happened shortly before GCU got axed...
RDR2 was completely removed, despite (IIRC) being available to preorder for a long time before. If Rockstar felt like they were a big enough deal to stand up and say "stop devaluing our games or we won't let you sell them" then how long would it have been before EA and Activision pulled the same thing?
BB saw the writing on the wall that publishers weren't happy with it, and GCU is meaningless without games to sell, so that was also a huge factor in why it went away.