Noted and memorable flags (Japan, Canada, Switzerland, etc) use single elements with bright colours. Our new flag needs to be identifiable when small (eg on the web), thus an uncluttered design is essential.
Why a fern? The fern (like the maple leaf etc) is not unique to NZ but NZers have made it so in many many ways.
NZ is now very multi-cultural so choosing a koru or similar ethnically-related element might not fairly represent the many other cultural backgrounds NZ now has. A symbolic kiwi shape, while exclusively ours and thus very identifiable, could be a second choice but doesn't lend itself to reasonable design.
So ... a single primary element - the fern in white - surrounded by red and blue. All three colours are a nod to history, our current flag, and blue represents the sky and the sea, while the red represents our land, especially our volcanic past which formed our islands. Credit for the fern design goes to Kyle Lockwood.