I guess it's mostly because cpython has a fairly good C API allowing PyO3 to just write "safe" wrappers on top of cpython APIs and provide macros to generate boilerplate whereas wasm-bindgen has to generate both Rust and JS sides and deal with the painful linear memory intermediate.

I think its a large part just the tooling. wasm-bindgen hasn't seen active development for several years at this point.

The good news is that since August 2025 there's been quite the uptick in development, in large part thanks to folks working over at Cloudflare.