I've been wondering about this for a while and haven't really found a great answer for it. From what I understand, WASM is:
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Faster than JavaScript
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Has a smaller file size
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Can be compiled to from pretty much any programming language
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Can be used outside of the browser easier thanks to WASI
So why aren't most websites starting to try replacing (most) JS with WASM now that it's supported by every major browser? The most compelling argument I heard is that WASM can't manipulate the DOM and a lot of people don't want to deal with gluing JS code to it, but aside from that, is there something I'm missing?
From a historical standpoint, there is also the bad blood of ActiveX, Flash, Silverlight and early Java applets that still leaves a bad taste in people's mouths. It has a slightly steeper uphill battle to fight.