1. This is gonna be unpopular, but: the target OS X users for this low/medium priced softwares are more willing to pay instead of pirating, than their counterparts on Windows.
I say that based on empirical assumptions, I have to say, but what I notice is a general cultural difference on this point.
2. A huge part of the web creatives community uses Macs, therefore it makes sense for these devs to focus on OS X
3. I don't know about Sketch, but for Tumult the reason can be found in their official "about" paragraph:
"Tumult Inc. was founded in 2010 by ex-Apple employees whose mission is to make the next generation of tools to produce beautiful web sites and apps. Tumult Inc. is funded by Y Combinator"
About #1, totally agree there is more piracy on Windows than OS X but I am not sure it makes a difference in the number of sales. i.e. a lot of people will pirate it but isn't that the ones that would have not paid for it in the first place?
Also, single platform toolkits are just so much nicer to use compared to those working on multiple platforms.
