Usually because it's a 1 way street. It's much rarer for girls names to be used on boys so it doesn't seem fair. Once a name starts being used more on girls, it won't be used on boys as much anymore, because it will be seen as too feminine.
http://www.behindthename.com/pnl/72895
"It is human nature to think wisely and act in an absurd fashion." ~
Anatole France