From the airlines point of view if everyone is taking more luggage why should we have to pay the price of extra fuel to cover it? if everyone brought on 5kg extra imagine over 148 or 235 passengers how much it would be?
And at the end of the day... its another money maker for the airline
I used to work for a handling agent and the airlines would pass down targets and also compare u to other bases and the best base usually got prices and so did the agent with the most excess chagred.