Earlier quoted context omitted.
They "why" is basically having a language with the aesthetics and usability of Ruby, but fully statically typed with global type inference, and impressive performance. The downside being a relatively slow compiler. It's the necessary trade-off for the type inference.
> The downside being a relatively slow compiler. It's the necessary trade-off for the type inference. I'm curious about that, MLs usually have fast compilers (OCaml for example) and have type inference. I thought that the slow compilation times were due to LLVM .
I believe you can speed up your compile times a bit by being explicit with annotations (which I often prefer anyway), but there's still a lot of overhead for the global type inference.