[turndown]

I think questions like this are both ultimately unknowable and assignable to a few realities. Rust is newer and thus was more likely to gain steam, all other things held equal. It was built for a specific reason, in response to a specific desire(a concurrent CSS renderer for Firefox?) so it was focused and useful in a difficult context. It featured many novel or under applied(in mainstream programming languages) features that solved annoying issues. Rust has enjoyed relatively few newcomers to its space since its inception. But on inspection many of these things might not matter at all(except the first one, IMO) and the truth may lie in other communities(as in, C++)