basicly because AMD not provide virtually NO support for GPGPU developers[both software and hardware] for years !!
even datasheets findings is troublesome[and many things undisclosed yet :/].
and Nvidia actually co-operate with devlopers and even co-assist in development itself
i whish AMD start investing little more in building healthy eco-system around their fellow GPGPU developers/enthusisasts :-|
starting from improving OpenCL related SDK, expanding devs support division and helping in making 1st GPGPU steps.