Developers write code for platforms that they're using. This is what drove adoption of Apple hardware in the early 2ks (a POSIX that runs MS Office!), which in turn set set the stage for the iPhone and iOS.
Unlike then, Apple now has a good grip on the consumer market: Not targetting iOS with a mobile/tablet release is a bad idea, regardless of whether or not devs are familiar with it. But OSX? There are a few areas where it's still has strong devotees: Color management in OSX is still fantastic, creating a lot of loyalty among artists, photographers, etc... It's still technically a POSIX, so it's still attractive to developers.
In short, OSX is not targetted at consumers. When someone needs a "computer" for their kid to do their homework, that's increasingly going to be an Android/iOS device.
For a platform to survive, it needs a healthy developer community. A developer community needs incentives (e.g., market-share or devs already using the platform). Right now, what are the incentives for OSX devs?