why does LCC operating from the same airport have to pay less than a traditional carrier
Stuck - I think you already answered this. It's because Etiuda is overcrowded, barely able to cope with the volume, and not a nice place to spend time. If I've paid very little for the flight I'll put up with grotty conditions. If I've paid a premium price for the flight, I want the airline to provide a quality product and the airport to be a comfortable environment.
Much the same as flying long-haul first class with the nice lounge, or with no lounge in economy - you still fly from the same airport and the flight takes the same amount of time.
Of course, each airline at Warsaw has to then ask itself if it wants to promote itself based on quality or on price and act accordingly....