A decade seems long enough to get the message that your parents act and react in certain ways and are very likely to cancel at the last minute. I do not say this to excuse them for not showing up at their own granddaughter's wedding, which I agree is inexcusable unless they really were ill or unable to travel on their own. I say it to point out to you that the time has long since arrived in your parents' lives when you may need to take more of an active role in caretaking for them. If you choose not to do so, then you can only blame yourself if your parents continue to disappoint you.
Keep in mind that I am not saying that you should treat them as children. I am only saying that you may gradually need to take more and more responsibility for seeing to it that they meet their obligations. If they agree to attend an event, make arrangements right away for their transportation (rather than waiting for them to cancel at the last minute on the grounds that they are unable to travel on their own). If you show up to take them to an event and they claim they are ill, treat them as if they really are ill. "Oh, I'm so sorry to hear that. I'll be happy to make an appointment for you with your doctor first thing after the wedding. In the meantime, I'll make sure the bride and groom know to stop by to visit with you after they leave the reception."
Basically, the way to honor your parents under these circumstances is to accept that they are getting old and need more active caretaking. If they protest that they do not need you to give them transportation or take them to the doctor or such, then you can point out that you know that they would not deliberately hurt their family by not attending important events unless they truly were ill or physically incapable, and so you all need to sit down and figure out exactly why they have been unable to meet their obligations.
God Help Me! These People Are Driving Me Nuts
by Gregory K. Popcak
When Difficult Relatives Happen to Good People
by Leonard Felder