When you leave the airport you are in the countryside in 5 minutes, on holiday. In Auckland, in 5 minutes you are in a traffic jam.
It is cheaper as rental car companies want their cars brought north again.
It looks nicer due to the way the light falls when viewed looking north
You start in the most empty part of New Zealand and travel to where there are more people so arriving in New Zealand is amazing straight away and coming home is not such a culture change.
On your last day, you don’t have to be up early to catch a flight to Auckland to head home. Instead, you can enjoy a leisurely breakfast and an easy drive to the airport and start your trip home feeling relaxed.
If anyone only offers trips that start in Auckland and finish in Christchurch then they don’t know what they are doing and shouldn’t call themselves a New Zealand specialist. They can call themselves a sheep.