Why is kiwi most different to the other birds?

Their unique characteristic is that they have nostrils at the END of their beaks - which seems to go well with their highly developed sense of smell.

Also

Unlike most birds, the kiwi is flightless. It falls within the infraclass Paleognathae (i.e. ratities) - a group of large Flightless Birds that also includes the Ostrich, Emu, Cassowary, and Rhea as well as the extinct Moa and Elephant Bird. It is most closely related to the extinct Elephant Bird.

They are also unusual among other birds in that, along with some raptors, they have a functioning PAIR of ovaries. In most birds, the right ovary never matures, so that only the left is functional.

Like many animals in Australia and New Zealand it probably developed so uniquely because it was ecologically isolated. (The Kiwi is only indigenous to New Zealand.)