The HTML output includes hard-coded styles that specify the green/red colors for insert/delete. This means that overriding the default colors requires a Python code change. Leaving the tags bare means that the color can be easily tweaked via CSS.
For example, see this template in django-reversion-compare, an app which recommends use of diff-match-patch.
Since pull requests are currently disabled, here's a patch in my fork of diff-match-patch