You stated it had to be profitable at a minimum. Now you're asking why it wouldn't. This is entirely my point - HOW DO YOU KNOW?
Answer is - you don't.
And here's where paragraphs help you understand big boy conversation.
First of all, you have staffing costs. Staff costs generally increase at a faster rate than hours worked due to overtime, union rules and industry award requirements. I'm not citing a specific park's bargaining agreement here, just speaking generally. Weekend casuals pick up some of the slack, but not always.
There's lots of other little costs that crop up during night events that aren't present during the day. The park may need to bring in or install extra lighting if they haven't, or don't usually use that part of the park at night. Special merchandise (and flashy crap) plus people to sell it. Extra security, Extra cleaning, extra maintenance, extra wear and tear... and then there's hiring in additional staging, scaffolding, PA and lighting gear for the special event stages.
But the biggest consideration by far is ride cost. Saying the energy cost is 'much less' because all the other rides aren't running is a bit misguided. Running the big 9 is the majority of your energy budget. Not running a chair swing isn't going to save you a lot of power when you're launching ToT's LIMs every few minutes.
I can't quote you every ride cost, as I simply don't know - but I do know ToT goes for about $1500 an hour, and TailSpin at about $1500 for an hour and a half for private events. (And bear in mind staffing needs at private events might differ from a 'capacity' public night).
Even if you assume a $1500 average (and i'm confident its far more) - 9 rides, running from 6:30pm to 10pm (3.5 hours) comes out a little over $47,000.
I did a little googling and found the family christmas after dark event had tickets on sale from $29.99. That'd mean you'd need to sell 1575 tickets just to cover your basic ride costs. I found a groupon style website selling park after dark tickets to another event at just $10 on top of a regular ticket. At $10 entry, you'd need 5000 tickets sold to break even - and this is just break even ride costs...
Park capacity is apparently 20,000 (which I find hard to believe), but general, full operation capacity, and night-time, reduced rides capacity is a very different thing. Cut out the wildlife area, Rocky Hollow, Wiggles World, Dreamworks Experience, and your park size is reduced substantially. You're basically trying to keep people in Ocean Parade, whilst allowing them access to giant drop and Mick Doohan - a heck of a long way from the rest of the entertainment - so let's say conservatively we stick with 10,000 capacity.
There is the potential that up to half of your total capacity is eaten in just your ride cost, without factoring in food and beverage staff, merch, entertainment, hire costs and so on. Now, you'd hope that your food, and merch teams make a profit, but if people eat before they come, or aren't interested in flashy crap, you could potentially lose money - so thats always a risk.
Bottom line is as I said before - they ran the ticket prices very lean, I presume, to attract more people to the park. Clearly the price point was too low, as they sold out and hit capacity - but what happened with the 26 May event? cancelled, and never rescheduled. If it's that profitable, why wouldn't you reschedule it?
Having said all that, the park website does suggest Park After Dark will return in late 2018 - I presume for Christmas or Hallowe'en events - so I guess for now watch the website and await announcements...