One of the big reasons that there tend to be fewer Apple based attacks than Microsoft based attacks is the fact that it tends to be more profitable to create one that targets systems with a larger market share. You saw this with Firefox when it first came out, as more and more people switched to this "more secure" browser, more and more hackers started to target it. With Apple systems gaining popularity, due to being "more secure", I suspect that we will see more and more attacks being targeted at these types of systems.