Why are so many quality apps/ programs Mac exclusive?

This has been a matter that has been baffling me for a long time. Why are really great apps, such as Fantastical, Clear, Sparrow and many more I can't possibly name available only on Mac?

At first, the logic of the mobile industry made a little sense to me. iOS has tons of users, the kind who don't mind shelling out a few bucks every now and then. Plus, the lack of fragmentation must be a big bonus. But then, the same logic fell flat when I remembered the pale number of Mac devices against Windows devices. Surely, more people must mean a larger target crowd? Also, since iOS has such a large market share, many, or maybe even most iOS users might have Windows devices too. If we don't mind paying for mobile apps, why wouldn't we pay for desktop ones?

Then came another thought: Piracy.

I'm not saying that Mac users inherently don't pirate or that Windows users are bound to, but the fact is, most Macs are pretty expensive. A person who feels that the device is worth paying for would surely not mind paying for some apps to make that experience better. What if the app gets pirated in Windows? But another counter came up: What about the free apps? Surely, great, truly great free apps like Cobook, Pocket, and quite a few others don't need to fear piracy when the official app itself is free? And what about indie devs? Surely, releasing Windows software won't hurt them a lot.

Also, one of the reasons for piracy is high prices. Most people wouldn't mind paying up $40 for the Windows 8 Pro upgrade, but I know loads of people who pirated their way to a $200+ copy of Windows 7 Ultimate. One might argue that Mac apps are in general highly priced so such prices are bound to prompt piracy, right? But what if the prices weren't so high? I remember reading an article where a Realmac executive had to defend the pricing of the Clear app for Mac, by saying that they couldn't bring the prices down to the iOS levels people were wanting because unlike the iOS market, the Mac one is way smaller and hence needs to be charged more to make the app a profitable one. But with Windows in the equation, app prices should take a dive overnight because of the market boom.

I mean, sure alternatives and rip-offs of all kinds exist, but nothing quite matches up the actual official app once you've had a hand at it. And its not about lack of apps in Windows, but about lack of apps which work with their iOS counterparts in Windows.

Hence, the question remains: Why are so many apps Mac exclusive? Is it easier to develop for Mac only? Or is there some reason that I haven't though of yet?