Some will have found they had bitten off more than they could chew and some have simply not had the time/opportunity. Some cards will have disappeared in the world's postal systems. I find it hard to believe that more than a couple of people, if any, will have signed up with the deliberate intention of not sending any out.
It would help if people who could not make the deadline this time around, for whatever reason, said so and sent them out in the next few weeks. Calling people names is not going to encourage this: it just encourages them to keep their head down and keep quiet.
Having said all that, there were by far the highest number of defaults I have seen in the last round: I got about a 60% return rate and the trickle of late cards I normally get did not occur - no doubt doubly annoying to those who went to extra trouble this time around with their cards. Hopefully, disallowing defaulters from further participation will reduce the problem in future.
P.S. As always, if anyone didn't receive my card who should have, PM me and I will get a copy out to you (it's in the Gallery in case you couldn't read my scribbled signature on the card!).