I feel CSS's layout model is simpler than (La)TeX's layout model. At the time Knuth developed TeX, the idea leading to CSS model is a "difficult" things? Or the CSS layout model is not appropriate to be applied to (La)TeX.
When Knuth invented TeX, there was no Computer Science, no web, no html and no CSS. My personal opinion is exactly the opposite. If html was marked as LaTeX life would have been easier, the web would have looked better etc. If there is one aspect that I agree with you is the lack of a
The current shortcomings of CSS/html can easily be observed by the fact that there are over 500 billion pdf documents on the web and the best way to create a pdf currently is with one of the TeX->pdf engines.
Note that the CSS layout model is not simple, because it includes a concept of gluing together boxes both horizontally and vertically, which is tricky. The apparent simplicity lies in how parameters are passed to the layout engine, through stylesheets.
Context actually does permit specification along the lines of style sheets, using its setups: to the extent that it is more complex, it is in the larger part due to being more ambitious. I seem to recall there was some work done on layoing out directly from CSS-like stylesheets.
The why is about ideas: the ideas about programming languages, document presentation, and abstraction that guided the design of CSS came after Tex. Both Context and Latex have aimed to reduce the coupling of layout issues from content. It's an ongoing effort.