Perhaps because they still believe in the promise of Web 1.0, where their app is a graceful-enhancement over an initial server-rendered document, that can be easily worked with at a DOM level by any HTML scraper, easily baked to a PDF and printed, easily re-laid-out for better readability just by changing the browser font size, easily text-to-speech'ed (including ARIA roles, alt text, etc), easily re-styled with a user-agent stylesheet, easily intermediated by browser extensions, and so forth.

I've yet to see a WASM-driven web application that's any less opaque to these technologies than a Flash or ActiveX applet would be.