I'm assuming because of the way an animal is sacrificed in order to be considered "halal". It would require a large slaughterhouse with the proper setup, which in an 98% orthodox country is not a easy arrangement.
About 25% of red meat exports from New Zealand are halal, and almost every slaughterhouse is capable of halal slaughter.
It's actually not particularly difficult to do halal slaughter, the two main requirements are that it is killed with a cut to the throat (common in non-halal slaughter) by a Muslim.
I'm not sure the muslim is a requirement per se, it's just that in practice there's an overseer. Many/most Muslim clerics accept kosher meat as halal, for example, even though the supervision is not necessarily Muslim.
It's actually not particularly difficult to do halal slaughter, the two main requirements are that it is killed with a cut to the throat (common in non-halal slaughter) by a Muslim.