Ask HN: What made VLIW a good fit for DSPs compared to GPUs?

That's not a real dichotomy. A RISC processor can range from completely lacking vector instructions to including complex matrix multiplications, and a so can a VLIW processor.

As far as why there are architectural differences between DSPs and GPUs, the largest reason is that DSPs are designed for processing one-dimensional data, like sensor readings or audio, where as GPUs are designed to process two-dimensional data, like pictures and video. Adding a dimension greatly increases the complexity of the data-processing algorithms the device will be running.