There are still problems with WASM in the areas of multithreading, shared data structures, and memory barriers (think advanced/concurrent GC implementations). Because of this, languages do not automatically, cleanly and efficiently map to WASM yet.

Most of the pre-existing server side come with their own baggage. PHP powers most of the internet, just saying :)

I don't have a problem using good WASM libraries that were written in PHP. To target WASM, a PHP compiler would have to respect the WASM semantics which is what I will consume.

For a dynamic language to run on WASM, you would have to bundle its runtime with your application, which is probably a deal-breaker for many use cases.