Basketball was a basketcase, they were constantly going bust and had to even pay to have the games televised, and the breakers were able to dominate simply by being well run and solvent. That success built an audience, also the Northshore event centre only holds a few k so it's easy to get a big crowd.
The point was that the Breakers had a lot of success early on, it was easier to be successful in the NBL when the Breakers were dominant because the whole sport was tinkering on extinction and most of the teams were under resourced or mismanaged. By being well resourced and well managed they were a cut above the competition. The salary cap in the NBL is a third of that of the A-League as well.
If the Phoenix started with multiple championships and playoffs and were one of the richest teams in the competition then they too would have a big fan base.