Because you generally get what you pay for. Also part of the process of the trial is to understand wear/tear rates and the associated costs with maintain that rifle time in service, so a cheap rifle up front could cost a lot more over time.Never having been involved in huge defence procurement decisions (I wouldn't be here with you deadbeats and losers if I was, I would be sipping Crystal champagne out of the cracks of Russian hoors on my yacht in the Med if I had been) I have to ask how complicated would the decision making process for the NZ armed forces' rifle be?
I mean honestly wouldn't it just have been a list of four or five tried and tested rifles that have been knocking around for the past 40 years or so and just picking the cheapest? What would be so special about the needs of the NZ armed forces that no one in the US, UK, Germany, France or even South Africa or Brazil if it comes to that, would have failed to spot in extensive use over decades? It's a service rifle, there's literally millions of them in use all over the world, just pick the cheapest and buy it New Zealand, whichever one you pick will probably do just as well or just as shit as any other one.
Doctrine will also have an effect with some countries will be happy with a rifle capable of hitting a barn door (or non compliant citizens) at 100m in the hands of a conscript when other will require effective individual fire out to 500-600m.
Nearly all the nations you mentioned have either just have or in the process of replacing their service rifles or have had to make a lot of modifications to their design to keep them current.