Because when waterlogged the xylem in lighter woods swells a little bit more, giving a nice grip on the puck and they compact well through impact - which means that instead of bouncing off (as with hard plastics and to a lesser extent jarrah), you actually get more surface area in contact with the puck and can retain more control. The reduced mass also means they move just a wee tiny little insy winsy bit faster through the tricky stuff. Oh, and they flick crap.
Now, I'm not talking exceptionally light woods here - my current sticks are made of Ash - but tasmanian oak and a few of the NZ natives are good examples of lighter woods that make good sticks. I guess I just don't like anything that's tooo hard.
If you can find something a timber merchant calls a hardwood that has a long grain, then you're probably onto a good thing.