Why do people make comments about not being invited somewhere and then, when they are, make up excuses to not come? Why?

I invited someone to my shower this weekend because she's a family friend of my husbands; in actuality she's a best friend of my MIL. In all honesty I forgot about inviting her until I was on the phone with Sean and she made a comment about not receiving her invitation. Anyway, she agreed to pick up Sean at the airport at 12:30 with Ian because the shower is at 1pm, so I've been calling my MIL trying to set up a way for them to go together, thinking about waiting until she comes back from the airport and we all leave at the same time in separate cars so she can leave if she wants, and also giving my husband a lecture because he told me that he needed her to 'do something' for him in the afternoon so she couldn't come (which annoyed me that he would do that rather than have her come to my shower).

Well, tonight in the midst of our conversation I started saying that I spoke to my MIL and she's going to call this friend to discuss her coming, and he finally says, "She doesn't want to come, OK? She doesn't like those things at all and she doesn't want to come! This was the perfect way for me to get her out of it without hurting anyone's feelings, but I didn't realize everyone was going to keep on about it. She isn't going to go!" 

OK then. It may be my hormones, but I can pretty much guarantee that she doesn't have to worry about being invited to anything any more.

The way I was raised, you go to events if they aren't your 'thing' if you care about the person involved just because you care about them. Last time I checked, things like that aren't about the you but about the person the event is for. I may be completely off base here, but I am ROYALLY pissed off about it!

Unfortunately, I'm the type of person who can hold a grudge for a long, long time, especially if my feelings are hurt. I may end up letting go, but I never, ever forget. Not ever. It's like the wedge holds a permanent place in my heart and I'll forever have it within close grasp. I know it only ends up hurting me in the end, but in a way I know that I'll be able to save myself from ever being hurt by the same people over and over again by keeping these events in the back of my mind.