Except XSLT dates back to the late 90s. WASM was not even a glint in the eyes of its creators. Mozilla even used XSLT internally to drive the GUI for their browser iirc. I think based on that there is a good reason it was incorporated into browsers.
The problem is that XSLT has so few users that the browser writers don't want to waste any more time on it. Those few people who wrote their site around it will have to either rewrite it to use something else or add some third party webassembly implementation themselves.