[amelius]

And what is the reason why an existing language and compiler doesn't suffice for 1 and 2?

For example, Haskell and GHC took decades to become what they are now (a powerful language and compiler). It seems quite wasteful to rebuild that work. If absolutely desired, Haskell can also be converted into combinator form, and vice versa, so why not use it? Seriously.

[cdcarter]

Because it's incredibly fun to work on? It's a brilliant VM once you wrap your brain around it, and the hackers behind it got to really try something new?

[implicit]

Judging by the documentation, Urbit looks to be less about leveraging pre-existing work and more about reinventing computer science and software engineering from the ground up.