All airlines overbook their flights because they know that a certain percentage of passengers are unlikely to turn up. I haven't checked, but I expect this is written in the Ts&Cs when you purchase the ticket.

But sometimes too many passengers turn up for the flight. Obviously the airline don't know this has happened until people start checking in for the flight, hence it's unlikely they'd be able to ring you prior to your arrival at the airport.

Flights booked on airmiles are the first to be cancelled when flights are overbooked. Again, I haven't checked, but I expect this is written in the Ts&Cs when you purchase the ticket.

This is what I expect happened in your case.