apple has a lot to do with it. Apple drove the early days of opencl, and pulled out some time in ~2012. Ian made a decision to cut opencl support at Nvidia at some point in time after that. Had apple actually made a proper GPU compute language standard after walking from opencl, it might not have been such a empty landscape. But alas, apple wasn't in the HPC business, which was the only place that actually made money from GPGPU.

I can't speak to why AMD is so massively incompetent or why they weren't a factor in driving a competitor.

A factor that might not be obvious is that in 2012 CUDA was mostly a commercial failure. Aside from some penetration into oil and gas HPC and things like physics acceleration in games, there wasn't much traction.