My only guess is they have a parallel skunkworks working on the same thing, but in a way that they can keep it closed-source - that this was a hedge they think they no longer need, and they are missing the forest for the trees on the benefits of cross-pollination and open source ethos to their business.
Same reason it wasn't when it was obvious Nvidia was taking over this space maybe 8 years ago now when they let OpenCL die then proceeded to do nothing till it's too late.
Speaking to anyone working in general purpose GPU coding back then they all just said the same thing, OpenCL was a nightmare to work with and CUDA was easy and mature compared to it. Writing was on the wall where things were heading the second you saw a photon based renderer running on GPU vs CPU all the way back then, AMD has only themselves to blame because Nvidia basically showed them the potential with CUDA.