I think vodafone took a longer term strategy when they decided to be so open to resellers. They accepted that as one of the incumbents they would have pissed some people off, better to get some revenue off them (wholesaling their connection to a MVNO) than none at all.

2degrees did an impressive marketing job, i suspect that even if they were only a little cheaper or even the same they would have still grabbed a fair few customers. Of course the competitive pricing helped, but im just saying the marketing was also a good part of it.

Also by reselling to MVNOs and working with 2degrees, there is now a larger pool of potential customers that can easily switch to them by merely swapping a SIM.