The passenger charge is probably not the foremost item on people's minds when it comes to choosing Knock or Dublin.
The actual fare or time and day of the flight weighs more heavily when they decide. Very often passengers decide when and where they want to go and then find a flight that suits their needs. Loyalty to an airport doesn't even enter their minds!
In general provincial passengers tend to put up with the hassle of travelling to the Capital City airport rather than vice versa. How many Dublin people would travel to Knock or Cork to save a few euro?