Why do Apple Macintosh's only have one mouse button? It seems useful to have at least a second one for options that aren't regularly used.
Its because Apple explicitly mentions in its Human Interface Guidelines that all software providers should provide all functions available with a single click and they dont see any use case for providing a right click.
That said it does support an option to have a secondary click as shown below which brings up the contextual menu
The general consensus was that the reasoning behind the primary design decision was to remove the confusion among the average computer user for remembering which button to click and therefore increasing intuitive usage of the Apple user interface.
To quote the article
One of the slashdots post also has this to say
All of this said,one of the big downsides of this single button option resulted in the headache of using double click which was introduced by Apple in 1984 when they introduced a single button mouse along with the first Mac
To quote this coding horror article
The mice that come with Macs today have effectively more than one button, a press on one side is considered the primary button and the other side the secondary button. The primary button is associated with selecting and dragging and activating (with double click), the secondary button usually pops up a context dependent menu.
Earlier Macs had only one button, supposedly less confusing than 2 or more buttons, but this proved fairly limiting. Two button mice were standard on early Windows systems, and software that relied on 2 button semantics (most software) treated control+mouse button as the secondary button on Macs. This required 2 hands, somewhat clumsy, especially when you're used to that second button on the mouse.
In 2005 Apple release the Mighty Mouse, their first mouse with more than 1 button, that had 4 effective buttons.