The main page is sorted by most recent "activity" on the question, where "activity" is any change that affects the question, including answering, commenting, editing (the question, answers, or comments), retagging, or changing status. To be able to ignore retagging or closing questions on the front page, I'd need to store a second date in the database for most recent "important" change, which would be answering, commenting, or editing.
Not saying this is impossible or even terribly difficult, just that it's non-trivial and not at the top of the priority list. Consider this a "maybe".
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