The lack of adoption is because it’s not a complete operating system.

Using sel4 on a server requires complex software development to produce an operating environment in which you can actually do anything.

I’m not speaking ill of sel4; I’m a huge fan, and things like it’s take-grant capability model are extremely interesting and valuable contributions.

It’s just not a usable standalone operating system. It’s a tool kit for purpose-built appliances, or something that you could, with an enormous amount of effort, build a complete operating system on top of.