Well one reason I could think of why is custom kernels. To really get the most out of your model performance, you will likely be optimizing the kernels you use for your layers, sometimes fusing them. A GPU can't adapt to that as well. The best you can do is use TensorRT to optimize for a speficic model of GPU, but why do that when you can create ex. the optimal CNN kernel in hardware on an FPGA? On a GPU you can only work with the hardware that came with the GPU.

That being said, this is in regard to processing, not necessarily scaling it up. And maybe it makes sense for inference, where it would be nice making a processor that is made specifically to run some architecture and which doesn't necessarily process things in large batches.

But for training, obviously nothing is going to beat a GPU/TPU cluster because of pricing and seemingly infinite scaling of GPUs. If money is not a problem you can always just buy more GPUs and your training will be faster. But parallelization will probably not make your inference faster, since the "deep" in DL refers to the long serial chain of processing, and that's where a hardware implementation of the optimized model makes sense.

Ideally, though, you'd want a TPU, not FPGA processors. TPUs are cheaper and you can use them for research as well.