[nikdaheratik]

1) HTML is derived directly from SGML which was used to handle similar markup issues in printed texts. One of the reasons why HTML was successful was because there were already similar tools in place from SGML that could be quickly applied to handle processing and correctness. Adding math would require building a number of additional tools.

2) TeX by itself doesn't resolve very well into the DOM tree which is what parsers use to organize the markup data internally. You need a markup language that is similar to HTML if you want to make it easy for browsers, parsers, and libraries to adopt the new standard.