Well girls are naturally more physically comfortable with one another (my gf being an exception, and I'm sure there are others). It isn't strange to see girls hug exessively, kiss (on the cheek), hold hands, sit on each other's laps, play with each other's hair, sit way too close to one another on a couch etc... so tickling is just a natural extension of that high comfort level. They'll even share beds when they have sleep-overs n stuff.
Guys tend to be more protective over their personal space, so generally speaking it's rare to see guys (past a certain age anyway) sitting too close, or snuggling up together on a couch or whatever... and tickling as a result just seems strange. Again, past a certain age... young boys tend to be more comfortable around each other; I didn't develop my own personal bubble till I was early teens, before then it was all about play-fighting, tickle-fights etc.
That's my take on it anyway.
