The reason why the Crusaders are essentially Canterbury is because of the state of the other provinces in the Crusdaers franchise. If Nelson Bays or Marlborough were in the first division prior to next year, I wouldn't have been surprised to see some players from them in the team. However, you have to look at it from the Crusader's point of view. Out of all the partner provinces of West Coast, Buller, South Canterbury, Mid Canterbury, Nelson Bays, Marlborough, and Canterbury only one team is in the first division, and two are in the second division. Picking players from the third division to play in the Super 12 would be a huge gamble; while both Nelson and Blenheim are a good drive away which would mean any players that came from them would most likely have to live in Christchurch during the Super 12, then go back home during the NPC. This is basically what happened with the likes of Leon McDonald and Ben Blair. They decided to take the chance of playing in the Super 12 and first division NPC to further their careers, and it has undoubtably worked.
Besides, the Crusaders aren't the only team that doesn't seem to pick Division 2 players. All the New Zealand Super 12 teams are guilty, and what seemed like a great way of providing experience to players in the smaller provinces (I'm talking Division 2 and 3 here) by putting them in teams with the bigger ones hasn't worked and has only served to usually make the bigger ones stronger.