Having less conventions saves them money on rent. YOU foot the bill for gas and lodging, but they still get a kick-back from the city because of the revenue generated. So you drive more which costs $$$, you must stay in a hotel=$$$, and you miss work and lose $$$. Then you contribute $$$ at the DC.
The Eldub at our KH had the service meeting part about the conventions. He kept saying " LANDMARK conventions" and "THE SLAVE." Then it was all justified by reading from the Old Testament, where JEHOVAH(tm) commands the people to meet at such and such place. Then he said that the main thing to do is to go WHERE YOU ARE ASSIGNED. OBEY THE SLAVE, uhhh, errr,mmm....Jehovah. WHY? Because they have told the resppective cities that there will be a certain amoount of delegates coming to spend money, thereby assuring a kick-back from the particular city.
One hand washes the other. $$$$