Here's a similar question: does a metal bell ring better than a wooden one? Vibrations in wood dampen out very quickly. Vibrations in metal last much longer. Certain metals vibrate better than others. Bats are made of aluminum, I think, which does not vibrate as well as, say, stainless steel, but still a lot better then wood.
For more information on the vibration properties of metals, read up on tuning forks.