Why don't many of the people who ask a question in MO use math environment for their formulas in the title of the questions?
Granted this is a temporary reason, but I and I suspect many other members use smart phones with old browsers, where such rendering takes time. This is tolerable for questions, less so for the front page. Also, question titles without symbols are more friendly to web search indexing, so lots of people outside MO can find and have a better chance of understanding the question with a good romanized title.
Before the migration to Math.SE last year, the front page of MathOverflow did not render math mode formulas. This may be a leftover habit.
Here's an old discussion: http://mathoverflow.tqft.net/discussion/1040/latex-in-titles/