You're right - a $3k k80 is generally inferior for most DNN applications than a $1k Titan X. The primary reasons that big companies use K80s have to do with achieving high computational density, and licensing issues, more than the performance of a single GPU. Sticking 8 Titan X boards in a machine is a bummer job if you want to pack them closely together. But for academic researchers, a quad Titan X box is pretty solid and quite affordable.
A quad Titan X is still $4k for the GPUs alone, and was only possible in the past few months - people might have wanted to get stuff done in the years in between the last generation and the current generation...