It is a balance of how much you know, your attitude towards others, and how much time you spend on the board. The ELITE mods watch you for a while, and then they pick you. That, or by recommendation by another mod.
A moderator gets to live out the repressed frustration of their inferiority complex due to their appauling social standing in their lives on this board, by acting in a role that gives them some form of imaginary power over imaginary others - which allows them to believe that their lives do have some, however insignificant, purpose.
<The above message was written with about as much sarcasm and spite as I could muster>