There are many reasons that make Apple OS highly reliable. I am listing some reasons here, that I know of:
1. The first and foremost reason is the cost factor. A Mac is costly as compared to any other PC, why? Because Apple OS uses high end hardware developed by IBM. Apple has recently started using Intel based systems.
2. Apple controls both hardware and software that needs to be installed on Macs thus reducing the problems arising due to software and hardware conflicts.
3. Apple doesn't require an antivirus because not many viruses and malwares have been created for Apple OS. Reason being the over all market share. Windows has the largest market share and is been used world over for a long time. Also, Apple uses a sandbox technology, that doesn't let any malicious programs to install without user's knowledge. But, if a malicious program is installed by the user himself, the system can get infected.
4. Unlike Windows, Apple systems do not accumulate unwanted files and registry keys over a period of time.
That is all I could think of as of now. But I would say that the future belongs to Linux based free OSs and they will definitely take over Apple and Windows and they will cease to exist.