I'm very confident that users of these runtimes do not care about the underlying Js engine powering them. Bun succeeded because it was compatible with node and required much less configuration to get a standard typescript and react app running.

I'm not suggesting it is a conscious decision point. Node has since done a lot to catch up on "less configuration" but Bun still seems to be growing even as what Bun does well Node starts to emulate. Runtime performance is mentioned in this thread here and elsewhere and while some are attributing that to Zig I think some are overlooking the different runtime experience of JavaScriptCore. It's also the "only" runtime difference between Deno and Bun if you assume that Rust and Zig are similarly performant native layers for some saying they like Bun's runtime performance better than Deno's.