[eddieh]

Because Mac applications are more than just rendering APIs, its about the windows, menus, controls, and a complete cohesive UI that works like all the user's other applications. As a Mac user I'll always pick a Mac only app over any cross-platform app.

The second reason is that Mac users pay for software. You can sustain a business as an independent software vendor by targeting only Mac users. Linux users tend to want all free & open source software. Windows users tend to steal apps. A disproportionate number of Windows users I know are happy to steal all their commercial software—I'm not just talking about students and economically disadvantaged people, I'm talking about 6-figure salary software developers.

By building a cross-platform app you will decrease the amount of paying Mac users while increasing the number of Windows users willing to steal from you. The amount of work required to support Windows and all the missed Mac sales may not be worth the effort.