The reason for this is: The lower the weight of the aircraft, the less fuel is consumed.
Therefore airlines who advertise cheap prices try to fly with as little luggage and kerosene on board als possible.
Because both are heavy in bulk and cause more fuel to be consumed. And this is becoming more expensive since the crude oil that is required for production is a fossil and thus continuously runs out of commodity.
In order to save as much as possible and thus to make the highest profits the aircraft may be light weighted.
Now you can decide as a passenger whether you want to fly with a conventional carrier such as »Lufthansa« or with a low cost airline like Lufthansa subsidiary »Germanwings«.
At Lufthansa ticket innately costs more since more weight and larger dimensions for hand luggage is allowed in general.
At Germanwings you pay a lower ticket price but less luggage ist accepted and it is determined that any additions have to be paid.
Someone who only flies for a short trip and will not take a lot of luggage, is normally better off at the low cost airlines.
Who goes on a long journey and wants to travel with more luggage or value on service, will get away with more conventional airlines.
Of course there are premium, first class or business class tickets where it is permitted to take fundamentally more luggage with you, but these are also much more expensive.