Its the density of the wood*. And Stradivarius may have been lucky to have access to it!
Scientists say the patterns of the grain are markedly different from modern instruments. It is believed that the seasonal growth of trees in the early seventeenth century was affected by a mini-Ice Age. Stradivarius had the benefit of wood that was produced in conditions that have not been repeated since then, the journal Plos One reports.
* This guy already knew that. What's good for the guitar's good for the violin. Or something like that.