A promise is a debt with your name signed on it.

Saying you will do something transfers weight from you to the other person. They stop arranging a backup, they plan around you, and from that moment the thing is owed rather than offered. That is why a broken promise lands harder than a refusal would have. You did not only fail to help, you removed an option they had.

Be slower about saying yes and faster about saying when. If something changes, tell them as early as you know rather than as late as you can get away with.