Typical client programs intended to run on nascent versions of those architectures were not intended to be delivered and installed very often. In contrast, web pages might get changes deployed to production multiple times a day. Needing to deliver a compiled runtime solely in order to run your client side code is going to be a nonstarter for the overwhelming majority of developers.

Now, at best, we can hope that many developers will collaborate to make caching easier by agreeing to only use one specific version of each runtime, delivered from a single well-known source (though experience with e.g. jQuery means that we shouldn't hold our breath). In the meantime, extending WASM to obviate the need for delivered runtimes will even the field between Javascript and every other managed language.