Linux users tend to give much better bug reports than Windows users (if they do at all). That alone is probably a good enough reason to do Linux first. There are many more good reasons when the first goal is getting it functional and not getting as many users as possible (who will probably hate it if they’re not a technically skilled user because there will be bugs).
You’re making an assumption their first priority is the number of users. I would suspect that isn’t true, and they’re aware Windows has more users.