Hi There
All airlines seem to be being alot stricter with luggage weight this year.
The main reason for the excess baggage charge as explained to me was that the fuel that is needed for the flight journey is calculated on a weight capacity, Obviously the heavier the load that the plane is carrying the more fuel consumption is used.
I suppose as things are financially tighter for all airflight companies nowadays and more controlled,

they now charge and are alot more stricter because it was too much of a loss for the airline companies to continue turning a blind eye. Hence more fuel was needed for the flights, so more control on weight limits., which results in a charge per kilo.