I didn't sent out pre-RSVPs, but I can tell you that even the people who were all about coming a few months ago are now realizing they can't....and vice-versa.
I think it's hard for some folks when it's too far ahead. A lot of people's employment situations have changed, so 6 months ago maybe they could/couldn't, and things are very different for them now.
I am also finding people's knee-jerk reaction is to say they'll come, but once they actually start crunching numbers they see the reality and have to decline.