No B2B, customers are likely pre-college grads, and the pool of customers are tiny.
Anyone like to hypothesize why a company would be exclusive to Apple?
Beyond that, and to your actual point, different developers have different motivations. Maybe their tool fits a specific need on macOS or within their community, or maybe they are starting with what they know, with a view to expand to other OSs later if they get traction. Or they aren't trying to sell the most software, and so don't feel the need to fish in the biggest pond. Or their software is iOS specific, which is potentially more portable to macOS in a way that it might not be to other OSs.
Seems like lots of reasons, and probably not that different to why some software is only available on Linux.
Does enterprise use this? Any big companies?
Interestingly, its mostly the admin/production/HR team who are on the Macs. Additionally all company laptops are MacBooks.
Can you explain what you mean by this?
but I don't get 'pre-college grads'. i can't think of a single mac (or macbook) owner I know who is a non-adult (I'm presuming you were meaning to indicate a youth-angle, not education level). I'm sure there are college-age mac owners, but all the ones I know are adults, working for businesses (or themselves as a business).
it's also easier to support just one platform than many.
"there's riches in niches" may be the general answer.