It's simple really.
The operators of the boat have the following going on:
1. They probably have families and lived to tend to, I'd imagine if they HAVE to be away from them at night TOO, it needs to be worth thier while. It's not like they stay there 24/7 with nothing better to do than wait for divers to decide they want to grab a night dive.
2. Not nearly as full a boat as day time weekend operation.
3. Income -v- expense. Cost of operation, paying the crew, wear and tear on facilities, compressor, galley, etc. Everything, costs money.
4. They don't do this for free.
While it's a service industry, it's not all cash-n-bank. In reality, night diving while alot of fun, doesn't attract the masses as day dives do. So, when an operator does throw on night dives, and you're paying 2 tank prices for one tank night dives, they are really trying to make the boat actually make money. No one goes into business to lose money or break even. That would simply be insane.
One can't argue that "If they charged the same prices as day trips they would have more business". This is simply not true. Divers like anyone else, can express that intent, but it's simply not true. Divers too have "life happen" and not everyone is single and or jobless with nothing to do the next day. There arn't enough divers within any locale that are not only willing to go night diving off a boat with the frequency that will make lower prices or greater number of dives -v- price possible, but comit to it.