Why is 6 the number of confirms that is considered secure? Why is 6 is the number of confirmations that is considered secure? I haven't found any mathematical explanation or otherwise that explains why it is 6 and not 5 or 7. Is there a historical reason for 6? Is there a specific way to calculate it so 6 is just a number that was chosen?
I believe this is addressed in page 8 of Satoshi's Bitcoin paper when showing the probability of an attacker catching up.

The wiki states: 


  "Only 6 blocks or 1 hour is enough to make reversal computationally impractical."  


The key word is "impractical".  

The important sentence in Satoshi's paper is: 


  "Assuming the honest blocks took the average expected time per block, the attacker's potential progress will be a Poisson distribution".  


So, it is not that someone couldn't create the worlds first and own a $50 million ASIC farm, so as to have the ability to overtake the blockchain from a fork six blocks prior but that doing so would be a.) a massive undertaking, b.) obvious to all and c.) terrifically unprofitable to the attacker.