Many people thought the Mac was invulnerable. That's never been true, but the low number of viruses and attack attempts has made it feel true.

Many think that the Macintosh operating system is impervious to viruses or these kind of security threats. It is not that they are impervious; they are targeted less.

There was no market because there were no Mac viruses.

Mac OS X is not invulnerable. It, like any other operating system, has security deficiencies in various aspects of the software. Some are technical in nature, and others lend themselves to social engineering trickery. However, the general architecture and design philosophy of Mac OS X, in addition to usage of open source components for most network-accessible services that receive intense peer scrutiny from the community, make Mac OS X a very secure operating system.

Any operating system is impervious to viruses. It's just harder to do (to a Mac), it doesn't mean it can't be done.

They're gaining increased interest from the virus (writing) community. And as these devices become more pervasive, they become a bigger target.