Because I forgot to go.
This is a first. saintpookie reminded me at breakfast, and I remembered when I got to work, and then I thought about it again just after lunch, and then I just... forgot. Until 5 (two hours after the appointment), when I suddenly said "Holy crap, I forgot my dentist appointment!" I can't even blame it on a meeting or anything, I just spaced it.
I know this kind of thing happens a lot, but still. I feel really lame.