I would love to make Mac applications but the prospect of having such a small potential userbase makes it a non-option for me. Imagine, 10% of desktops have Mac OS, of that only 20% will buy my application; that's a very small user base.
20% of 75 million is 15 million. 15 million copies of any paid app is huge.
Also, let’s not forget that Mac sales has been growing for years, while the PC market as a whole has been shrinking. Today, there are 3 times as many Mac users than 5 years a go.