Because if you’re doing things properly you’re serving HTML gzipped anyway, so the low-hanging fruit of HTML minification – whitespace – isn’t all that relevant. There aren’t lots of easy targets (e.g. variable names) for minification in HTML, which are present in CSS and JavaScript. Much of the content of HTML is the actual content of the page, which probably can’t be minified (and, as others have pointed out, will almost certainly vary more frequently than your CSS or JS).