10. So Roger Daltry couldn't easily swing it around by the cord.
 9. To keep Brittany Spears from bumping the mic stand with her breast implants.
 8. To keep Tiny Tim from spitting on the diaphragm on the high notes.
 7. To keep it out of Barbara Strisand's reach, so she can't throw it when she gets mad.
 6. Keeps Rod Stewart from grabbing the mic stand during emotional parts.
 5. Because Sinatra did it.
 4. It was necessary in the old days, when people actually read sheet music.
 3. Keeps mics out of the way, since good mics are not easily replaced, while most singers are.
 2. Lets engineers stare at female singer's breasts without the mic blocking the view.

and the number one reason why "Large Diaphragm Mics Are Used Upside Down": 

1. Dyslexic Engineers.
(Or, because Sinatra did it.)