Why would something be only available for Mac? What are they using?

In the early stage of a startup/product the most important thing is usually fast development loops, to add features and, most importantly, figure out what the product needs to be. It usually makes sense to focus on that, which means deferring later projects (such as porting to other platforms) which are also important, but aren't part of the figuring-out-the-product loop and so can come later.

This of course is frustrating for users who want the product but aren't using that platform. But also a good sign that someone wants the product enough to complain about this!