I suspect it was because containers sufficed and using and creating the tooling around them consumed the attention of those who might have otherwise looked at unikernels.
I think you might be right. Right place at the right time for containers.
The lightweight-ness argument is enticing, but I'm wondering if the fact that now you have to give these VMs enough RAM to run the app won't mean that you end up with worse flatpacking in terms of RAM than if you used containers?
It looks like around 2015 there was a lot of hype around the topic, but as I tried to document the state of the art, I noticed there hasn't been that much pick up yet. What's your take?