There have been countless studies by people trying to discover just what makes Stradivarius violins great, and I doubt anyone will ever be able to put a finger on it with any certainty. There are just too many factors that go into an instrument, and even two violins made of wood from the same tree (well...usually it'd be spruce for the top and maple for the back and sides) won't necessarily sound at all alike.
It's probably a good thing they can't figure it all out.