Manuka Honey is unique to New Zealand and comes from the Leptospermum Scoparium tree, which is a native and limited resource. With a small window for harvest and difficult to extract, it is a specialised task for beekeepers to obtain quality Manuka Honey.
There are only 2800 tonnes of Manuka Honey (2018) produced each year, this is compared with 1.9 million tonnes of other honey produced globally. This 2800 tonnes has to supply the entire global market. As there are different grades of Manuka Honey the higher the grade, the less there is available and the more scarce it becomes.
There is extensive research supporting the health benefits of Manuka honey due to its incredibly powerful properties and for this reason, it is in high demand.
You can read more about why Manuka Honey is so expensive at this article:
https://newzealandhoneyco.com/blogs/honey-articles/why-is-manuka-honey-expensive