A large part of it is that those who can make money out of OpenStack are doing so, and those who aren't are doing the math and moving on to the next thing.

Take a look at the sponsors for KubeCon and Dockercon and consider that many of these same companies were those originally trying to catch the OpenStack wave, and then ponder how many of them will be extracting themselves from those projects in 5 years (or at least lowering their investment, which as we see here is generally perceived as extraction).

Prominent folks in the Kubernetes community will tell you that it's different but the recent conferences sure have a similar vibe to OpenStack summits circa ~2013 when the marketing really started to overtake what the technology could deliver.