Familiarity

Everyone knows Unix. Most college graduates are brought up on Unix. The effect is a large grassroots support network of users who have had the same problems as you before and can help you fix them.

Reputation for Stability

Unix doesn't die. People regularly compare how long their server has been up vs how long someone else's server has been up, even going so far as to buy UPS units for this.

Cost

Most of the Unices are Free, but even commercial flavours have quite fair costs per CPU as compared to other OperatingSystems.

; Transparency :

; of processes

You can learn what every process is there for, what implications removing it has, or which alternatives for it there are. For example, it's common to run a Unix machine with just a WindowManager instead of a full DesktopEnvironment, or even entirely without a GUI, if it doesn't need one, thus saving plenty of resources.

of files

It is easy to find out what any file is needed for. Easily viewed and edited plaintext files are common.

of source

Traditionally you get at least a large chunk, if not all of the SourceCode of your Unix system. You can always read it if in doubt. (And even lacking source, strace(1), and ltrace(1)? will both help you debug problems.)

Plaintext configuration files

Due to this format, configuration files are easy to search and manipulate with a host of standard tools shipped with Unix by default. For example, you can put your configuration files under version control.

Powerful scripting

Just about everything can be scripted. In fact a Unix OperatingSystem consists mostly of a loosely held together web of scripts. If a program doesn't support scripting natively, you can usually fudge it anyway.