Because there is no logic behind it. The correct way to say a current date, long form, is to say "it is the 14th day of May in the year 2016". That's why when we - users of the correct format :hey - use the long form date, we write 14th May 2016 and say "14th of May 2016". Amercans say May 14th (May 14th day of, makes no sense, does it?) and, as such, strangely change the short-form date format to suit. Much like many American disfigurements of the proper English format (I'm looking at you, people who say math instead of maths), it has no basis in sense.
On a computer now... If I said "5/15/16", I am NOT saying "May 15th day of", I am stating the month/day/year. Therefore, it is NOT wrong, just in a different order. That's like saying the left side is the proper side to drive on. And I guess we should abandon lbs and start using stones. So on and so forth. I have yet to see someone say "their culture is correct and mine is wrong" because truth is, none of us are wrong. I am not calling your way wrong and mine correct, just different. And I don't call other cultures or educations or whatever into question because I understand they're all different. I don't believe in a right or wrong, I am very open minded and respectful of others and I'd like to see others be the same. :hi: