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 stitching together boxes both horizontally and vertically, which is tricky (1) . The apparent simplicity of CSS lies in how parameters are passed to the layout engine, through stylesheets; by contrast, Tex uses a famously hard-to-master programming language to associate these parameters with the objects in the layout's domain model.
Also, there is one universally recognised reference implementation for Tex; CSS versions one and two were introduced during periods of intense fighting in the browser wars (2), which made it easy to produce some sort of document with the CSS2 box model, but far harder to ensure a good reader experience that with Tex.
The other important, growing Tex-based typesetting platform, Context NG, actually does permit specification along the lines of style sheets, using its
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.
(1): I say a bit more about comparative complexity in my answer to What are the differences between the CSS and Latex box models? at Stack Overflow
(2): Read Nicole Henning's from-the-library-trenches account of the browser wars, from the perspective of MIT's efforts to promote internal usage of web standards. Now, with CSS version three, there is no dominant browser; instead, the HTML5 and CSS3 web standards represent a sort of peace treaty.