The short answer is that selling developer tools is hard. Software engineers especially do not like to pay for stuff. We used to be in the DevOps space [0] and it is something that requires heavy enterprise sales to monetise well. There are few companies that truly took off in selling digital shovels, one example is probably Jetbrains. Most developer tooling/platform companies either died or got acquihired once they run out of funding. Flynn and Deis were both acquired and I suspect even the new generation of PaaS like Vercel would have to look for alternate sources of revenue once the VC funding dries up and if they do not achieve profitability. The container space is extremely challenging if you are competing directly with companies like Google with tremendous amount of cash and a willingness to buy developer mindshare. Docker may have popularised containers and singlehandedly brought the ideas of BSD jails mainstream but profiting from it is another thing entirely (you can do pretty well consulting on Kubernetes etc. but that would hardly give unicorn levels of return).

[0] https://news.ycombinator.com/item?id=23254394