Natural selection... those animals best adapted to their environment are the ones most likely to survive and reproduce. Those who reproduce pass on their genes, both for morphology and behavior. Therefore... if rattlesnakes who make noise are more easily detected and more likely to be killed, they are also less likely to survive and reproduce. If they are less likely to survive and reproduce, then rattling snakes become less and less common. The real question becomes, is this true, or is data just lacking?
Interestingly enough... there actually is a species of rattlesnake that does not possess a rattle. A coworker of mine used to work on the island where these snakes occur. I first learned about them from her.
Santa Catalina Island Rattlesnake: http://www.sdnhm.org/archive/fieldgu.../crot-cat.html
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