The other key factors for EP as a day night test venue are:
a) The lights - only EP and Westpac stadium have sufficient lights to host a day night test in NZ. Even then they aren't quite as good as Adelaide oval's lights. Westpac isn't considered a test cricket venue by NZC.
b) The corporate hospitality facilities. This is a huge factor for NZC as the playing times allow for far more corporate interest. That's a big money maker for them which they don't get at other venues - they are happy if it's just one test a summer that they get to do this.
c) The shape and size of Eden Park is considered less of a factor for test cricket than white ball cricket as the majority of the important instances in tests happen in and around the pitch.
That said, I agree EP is not an ideal test(any cricket even) venue and Auckland desperately needs something better but for now while it's all NZC have available in the most populated city in NZ, it will be used occasionally.