there are different fiber providers (the companies that install the fibre) for different areas of the country, tauranga for instance uses Ultrafast Fiber, Auckland uses Chorus, Christchurch uses enable. (note there may be more than one fiber provider in those areas mentioned I've just named one)
the ISP needs to have a deal with the local fiber company to be able to provide services over the fiber companies network. in your case the 2 you mentioned have deals, slingshot doesn't.
They are probably working on it but how long that will take only they will know.