I think people overestimate the drive to go pro in a lot of guys playing in the prem. I get the feeling there are plenty of pretty good players that have given up on that dream and moved into careers where they see a long-term future and don't want to disrupt that when they can still play at the top level in this country. Others have been pros before and might just be more comfortable where they are. It takes time for your body to adjust to the higher pace, so you're unlikely to be in the first team frame for the first few months, and you would have to really believe you are capable of moving up from last in the pecking order to the bench or the first 11 if it's going to make it worth your while unless you really value the chance to be professional footballer that highly.
I think you pretty much nailed it. Probably a combination of different factors - and different for each individual.
Rudan has stated that he was interested in certain Kiwi players, who were prepared to come to the Nix but didn't want to trial. So on that basis doesn't appear that the money the Nix were prepared to offer, was the sole problem. Howieson & Payne are the names most often bandied about. They have both been recent AWs, and both have been conspicuous by their absence as trialists in Wellington.
If you look at Howieson (Burnley & St Mirren) and Payne (Blackburn & Portland), both tried pro football and both have effectively just been fringe squad players. Maybe they just both good sick of that grind of having to prove themselves all the time, with a high level of uncertainty month to month. Similar in a way as to why Kip Colvey gave up pro football.
At ASB level they are some of the main players in their teams. They may just feel happier that way. As much as I'd love to have as many quality Kiwis at the Phoenix as possible (and I criticised these guys when they initially failed to trial) - on reflection you can't begrudge them, doing whatever makes them happiest.
The one factor that could change things, is whether part of their 'no trial' reasoning was that the Nix had stunk for a few years. The club's reputation wasn't great. Rudan & co have turned it around big time. Howieson & Payne may just be tempted to change their minds.
They will always know that they were AWs, and good "shamteur" players at a domestic NZ level. But in the right pro environment, could they have been better players? Could they like Sigmund, Krishna, Riera make the jump and become regular starters for the next 5 odd years? That might just nag away enough to make them change their minds.