Disney will wait until the last minute if there might be a chance that the rain will clear up enough to continue with the parade. I was there once when the CMs were telling people they were welcome to wait in the drizzle but that the parade would likely be cancelled. Many people left their spots on this advice. But in the end they didn't cancel it, but started it a few minutes late.
While I totally understand your frustration (we had FP last year and it was cancelled, that was double frustration!)....but the alternative is that they cancel it an hour before start time so that people don't line up....and then the weather clears and they could have done it, but hey, it's already cancelled, too much chaos to try to get it going again.
This is also why we tend to try to do the parades and fireworks early in our visit....so if something happens we still have time to try again.