Most people hack on this stuff for work, and time is money. OpenCL is just a lot less productive than CUDA for most tasks. The NVidia price premium isn't big enough to make people switch over.
Agree, been doing some cuda as part of my job since 2009, and it works very smooth, you could write non trivial programs after less than one week of learning (if you know C before). When opencl got a little hype, I had a look, I didn't manage to run a simple example, and I was asking myself: do I really want to have to use all this boilerplate code? Also at least a while back there is no good fft outside of cuda, maybe that changed though.