My guess is that, the number of people who:
- are booked
- decided not to travel and to bin the ticket
- know before leaving home that the flight is cancelled
- successfuly claim compensation
is very small.
Mostly, you only know about the delay once on site. The carrier usually wants to get you airside without telling you about the delay - as discussed in here many times.
The times when you have a bulk cancellation, such as Gatwick last Xmas, are few and far between.