Why do you invent a new language for your work?

Why did you not embed your language into another one, with type system that is superset of what you need?

For example, there's capabilities expressed in Haskell: https://github.com/tweag/capability

Capabilities there are tracked at type level and are subject to type erasure, if possible.

There are a few reasons for this. I really can't seem to wrap my head around Haskell in the wild as written by real people. I also wanted to tie capabilities into the build system tooling so that the build tool could verify caps and launch executables into an OS sandbox. Bolting on capabilities doesn't offer the same ability to enforce them, for example the March package manager ForgePM will reject packages that falsify their cap manifest. Moreover I was reading some papers that inspired the language, and wanted to try it. Cap(X) is also erased b the type checker during compilation in March.

I'll blow my own trumpet and promote my own Haskell capabilities library, Bluefin: https://hackage.haskell.org/package/bluefin

Bluefin is used in production, and as far as I know capability is not.