Thing is theme park Monorail systems (which lets face it is was Sydney's system was) are cheap as hell to run. That's why they're viable to theme parks which offer them free to guests. I suspect that the problem Sydney faced was no one could really be bothered with sourcing new rolling stock and dealing with implementing it when there are bigger fish to fry in the Sydney Transport network. I'd be shocked if it didn't run at a modest profit, but not enough for those who in charge of massive transport infrastructure to care.
At Sea World though, it is a big part of what the place is. At Sea World it offers guests an easy way to commute around the park, is a genuine all weather ride that the whole family can ride together, and more intangibly, it gives the park a kinetic energy as the trains cruise around. It is worth it to put the effort into it at Sea World.