Speaking of Yarn in particular since I maintain it, I strongly believe that more than speed we (package managers) need a good polish. And to do that, we need a community that feels empowered to make changes when they feel frustrated about something - which makes using JS / TS critical. By contrast, the UX of a bundler / linter is fairly straightforward (compared to Yarn's 46 builtin commands), so making external contributions easier isn't really necessary.