The Dolphins run juniors, i.e. people literally grow up playing against the Dolphins in most age groups.
Do you really think little jimmy who grew up playing for, e.g., the Morton Bay Raiders is going to have a high opinion of the Dolphins? Some would undoubtedly jump onboard a Dolphins NRL team, but to most the Dolphins will just be that rich club from the other side of town that pushed their club around and bought themselves into the NRL.
In other words you literally have generations of RL fans in Brisbane that have grown up seeing the Dolphins as another tribe, and that is really, really, bad for the Dolphins prospects of converting those people into fans.
If you brought in the Dolphins as the Dolphins then you'd have to wait for a couple of generations to grow up under the new paradigm of the Dolphins being an NRL club before that wasn't an issue anymore, but why would you do that when you could bring in the e.g. Brisbane Buccaneers and not have to worry about any of that shit because Buccaneers is a neutral brand that carries no historical baggage. They could even be owned by the Dolphins and it wouldn't be an issue assuming they played their cards right, but if you bring in the Dolphins, in red and white, and based in Redcliffe it'll be a shitshow of grand proportions.