Meetup provides several ways to update your RSVP, including the web site, mobile apps, and the mobile web application. Unfortunately, with such a large group, it's increasingly difficult to change everyone's RSVPs by email request. And if someone posts a "comment" saying they can't make it, that comment may go unnoticed resulting in a no-show.
If it looks like you can't attend an event, please log in and change your RSVP rather than posting a comment or emailing the host.