Points of fact in no particular order:
Duty Free generates more profit per kilo than pax.
Women have larger bladders probably because waiting in line is less an option for them than it is for men.
In airliner design, a good way to save money is to delete a lavatory. Duty free generates revenue; serving alcohol makes sure the alcoholics don't fly with someone else, and probably generates revenue as well. Besides, the less a lav is free, the less likely people will try to use it. So you keep them to the required minimum.
Load factors are very high these days.
So on LH you will have people standing in the aisle, waiting to use the head. It only takes one inoperative toilet to make that a crowd.