US has 30 year mortgages backed by government. New Zealand and Australia do not so the overwhelming number are either floating/variable or fixed for 1-5 years.
https://www.freddiemac.com/about and friends buy mortgages that meet certain requirements (called "conforming" loans) and package and sell them to investors. Since they will buy 30 year fixed loans, banks are willing to sell them, and investors are willing to buy the subsequent bonds created out of them because they're (likely) almost as reliable as 30 year treasuries from the US government itself.
If you try to get a 30 year fixed loan outside of the US mortgage market, it's hard to find with rates as low.
CMHC may not buy fixed rate loans in the same way.