Exposure is probably the big thing, but that is something every A-League team would struggle with. We're competing in pay against leagues like the lower divisions in Western Europe and some of the top divisions in the East. Most are struggling to even pay their players, some are in places which are dangerous and not pleasant to live in.
Wellington definitely tops most of the surveys about best places to live that I've seen. It also has the highest wages, statistically the happiest residents, the most educated residents, and holds second place in the Governments regular quality of life survey (one point behind Dunedin) - maybe some of the smaller regional cities might be seen as better, but only if money isn't an option. When you factor in lower wages and less jobs I doubt there is any competition to the cities in the Wellington region and Auckland.