OCaml is also pretty good at leveraging the C ecosystem, but that's not what people are looking for in a "garbage collected Rust". If they want something close to C they'll just use Rust. What they're looking for is something easier to write that still looks like Rust and having all the developer experience that Rust has. For this, the Nim ecosystem is way too small and can't leverage an existing platform. The same do apply to OCaml and Swift so it's not a Nim specific problem, it's just that Nim doesn't bring much compared to OCaml.
For what it's worth, OCaml is a great under-appreciated language forever dented by ugly (in my opinion) syntax and oddities like different operators for floats and ints. There's a good reason the folks behind the Reason ML fork changed things up.
Swift is another great language - forever dented by being tied hand and foot to the Apple holy garden and Objective-C legacy and runtime with memory-related keywords such as 'weak' and 'unowned'.
Nim (and perhaps D) are the closest in my mind to the "garbage collected Rust" in that they are not only garbage collected, but they are procedural-oriented, can write highly generic code and macros, and come with near-C performance. Given the "Pythonic Rust" title, another edge Nim has is its semantic whitespace, very Python-esque syntax.
