It is no doubt that if you take a look at a room filled with programmers about 80% of them will be using Mac, But why is this?
The Use Of Unix:
Mac uses Unix, a unix command line closely emulates your target server. The use of Unix and common software is loved by programmers.
The Build Quality:
The build quality of a Mac is second to none, I can vouch for this as I type away on my butterfly keyboard on my MacBook Pro. Programmers spend alot of time working on their Macs and most have pretty extreme standards.
OS X Supports Cross Platform:
If you want to program for iOS you have to use a Mac, but on a Mac you can also program for Windows. Where as on a Windows machine you cannot develop for Mac or iOS.