Between a bit better language design and superior support and tools, CUDA is way easier to do your work in. We've 4 labs that use CUDA in one fashion or another, none that use OpenCL. A number have tried it (also tried lines like the Cell cards that IBM sold for awhile) but settled on CUDA as being the easiest in terms of development. Open standards are nice and all but they've got shit to do and never enough time to do it, so whatever works the easiest is a win for them.
On a different side of things, I've seen less issues out of nVidia on CUDA than AMD on OpenCL for video editing. Sony Vegas supports both for accelerating video effects and encoding. When I had an AMD card, it was crashes all the time with acceleration on. Sony had to disable acceleration on a number of effects with it. I had to turn it off to have a usable setup. With nVidia, I find problems are very infrequent.
Obviously this is one one data point and I don't know the details of development. However it is one of the few examples I know of a product that supports both APIs.