Of course there are genuine reasons why people need to cancel last minute such as illness. However, many people cancel going to children's parties at the drop of a hat. I think that because it's not an adult event, they value it less. It is inconsiderate and rude to cancel last minute without a very good reason ( or worse still to just not turn up) because essentially it shows a lack of consideration for the host child and parents.
People hosting parties have put time and effort and money into organising a party.
Last minute, non-essential cancellations show people just don't care about the time, effort and money spent by others. Despite having said they will attend, they view that as no barrier to their own personal convenience at the last minute.
Yes, people live busy lives and sometimes spending part of your day taking a child to a party or hanging about at one isn't someone's preferred way to spend the day or inconveniences the rest of the family....so declime the invitation. The problem lies in people not being willing to plan ahead or essentially being unwilling to out themselves out to any degree to stick to a commitment/acceptance they have made.
People cancel because they are tired on the day, a better offer comes up, they forgot to get a present, they didn't think ahead to what would happen to the other children, their DH isn't keen on the idea becaue he didn't know about it before the day or day before, they don't like the thought of soft play, hanging out with other mums etc etc. These are all selfish reasons really and show someone totally putting their own convenience ahead of the fact they agreed to go to something. It is totally different to having to cancel because someone is genuinely ill.
Lots of people are flakey these days. They don't consider that accepting an invitation or the fact the other person will then have made plans and might have gone to expense any reason to be held to their acceptance. They feel they only have responsibility to themselves and their own whims at the individual moment. Adults are like this towards other adults and let people down at the drop of a hat, and view acceptances of anything involving kids as meaningless. It's sad, but it's true of many people. And if kids grow up seeing that as normal, of course they won't respect accepting invitations either.
I've had to cancel last minute, if I've had a migraine or a child suddenly threw up. Those things have happened but not very often. I take the view that once you've said 'yes' you push yourself to attend even if it causes some inconvenience. So I've said 'no' to better offers which came later, or I've had a day with lots of driving to and fro last the same place and done 20 more miles than if the party didn't exist, because the other stuff of the day had to fit in with the party. I've been bored in soft play or missed out on what could have been a great day to go out with the family as the weather turned out to be brilliant...because I'd space ores an invitation and that acceptance meant something to me.
Fortunately, huge class parties involving lots of kids are pretty short lived and never compulsory. You hope that the serial 'let -downers' are not your friends and if they are, you either have a word or they probably drift out of your friendship group anyway. It's a shame when the kid is a good friend of your child but their parent can't get their act together or is just pretty selfish and doesn't consider your child or your efforts when they do another last minute let down at the drop of the hat. Of course, genuine things do happen and we al accept that....it's when they are extremely frequent that people start to feel a bit undervalued.