It is most likely because she herself was constantly disappointed during her formative years. It is annoying for you, but a comfortable pattern for your friend, and without psychological help, it may be hard for her to alter this pattern.
You could abandon her, or you could find a way to detach yourself by lowering your expectations for this friendship. If she promises to do something for you, even to meet you for a cup of coffee, you can say, "Sure," but protect yourself by knowing, in the back of your mind, that this friend "nine times out of 10" is going to cancel on you.
One way to try to change the Promise Breaker is to help her to understand the consequences of your ignored pledges. Try telling her how it makes you feel --
"Of course, I'll understand that you're not in the mood to drive over, but I was really looking forward to our visit."
Perhaps she is unaware that this is a pattern rather than an isolated incident --
"Yes, of course, I understand, but do you realize this is the fourth time in as many weeks that you've backed out on something you promised to do with me?"
If she has always been there for you, through thick and thin, has only recently become less reliable, you might want to cut her some slack. You have to decide if this is a lifelong trait that will be hard or impossible to change, a temporary condition that will be short-lived, or something, if it does continue indefinitely, that you are willing to accept and handle.
The next time she promises something, try saying, "Yeah, right." When she gets angry at your sarcasm, explain that you are simply pointing out her habit of breaking her promises. Then reframe it in a more positive vein by saying, "Prove me wrong. This time, keep your promise."
(source: Jan Yager)