Mostly because of how frustrating it's been getting them back. First of all, why do people assume that it's okay to not respond by the response date? Seriously, my RSVP date was May 1. It just drives me crazy. And people who I know are coming were some of the ones not sending their's in! Grawr!!
And then there's this whole business of a plus one. We purposely did not have a plus one line on our invitations. In spite of this, we had one couple tell us they were bringing their two daughters (who were not invited), and I've had several people just assume that they're allowed to bring their boyfriend/girlfriend! I mean, it's not a HUGE deal because we've had so many people decline. But still. It's just rude to assume that you can bring someone else when the invitation is only addressed to you. I'm so annoyed with people right now.
Also, it sucks that a lot of the people I really wanted to come can't. And a lot of people I don't really care about are going to be there. Go figure.