Bill D - Straight answer..........

because William is far too formal for what usually turns out to be a true love......
Because the thought of singing about darling Hubert or my dearest Kevin somehow doesnt scan, There are a fair few Johnnies in songs too, and there are double entendres there I am sure! Per haps its the ie sound that does it!

Silly answer...Willies are lovely!

Cheers, Lucy