A lot of people were asking for a way to get through the fights faster by having the option to fight the bosses multiple times at once or as many times as possible with the combativity you have at the moment. That's why the x10 boss fight was introduced. It was never to gurantee a girl drop.

If x10 boss fights guranteed a drop, that's like getting the 3 girls for 6 kobans, the same girls that are also available in epic pachinko, in which to get a guranteed a drop you need 900 kobans. Why would anyone spend 2,700 kobans in epic pachinko to get the 3 girls when they can them all for just 6 in boss battles? If you want guranteed girls from the boss battles, maybe they should increase the price x10 boss fights to 900 as well then.