I've mentioned before that I just don't get people who blow off their friends. Can't you call at a reasonable time in advance to say you can't make it? Can't you realistically look at your schedule and your life before you commit?
A couple friends of mine were just blown off by people they were supposed to meet. One was an old friend that moved, and the other was a brand new friend they met at work. The variety in this boggled my mind. So it wasn't just the new friend that was clueless, it was also the old friend. In both cases, the person didn't call, didn't bother letting them know that they weren't going to show up. Rude.
Now, of course, these friendships are in jeopardy. Will they survive? Unlikely. I can't help but wonder why someone would continue a friendship like this if they didn't want to really be friends?
In the case of the long-time friends, I think social media did them away. They were fine when it came to emailing each other or hanging out on Facebook, but when it came time to actually be together, something was missing. Certainly at least, the commitment wasn't there to make it a "real" friendship any longer.
In the case of the new friends, I think it's a matter of fear. Perhaps the person wasn't sure if they wanted to be friends, so instead of taking some time to figure that out, they just blew off the chance to spend an hour or so getting to know someone new. My advice would be to just go, see if you like the person outside of the office and if you perhaps could be friends down the line. An hour or two to maybe have a friendship? That's worth it. Instead, the person blew them off and now they will probably never have the chance to really be friends.