I suspect it was to do with ease of publishing content, which when the Web began was mostly text. If the web itself was also text then it becomes a simple matte

I suspect it was to do with ease of publishing content, which when the Web began was mostly text. If the web itself was also text then it becomes a simple matter to write documents in your editor and push it (with a little markup). Converting text content into bytecode describing layout would need a dedicated program instead of using any old text editor. I don't think the creators of the web ever imagined how it would take off and the kind of scope-creep that would happen even in their wildest dreams. And now we're stuck with the existing formats because doing anything else will likely break billions of old websites out there.