According to scientific tests before audiences, the best violins made today sound as good as the best violins (including SOME Strads) made in the past.
The public is not aware of this because violin dealers can make more money by creating a mystique around old violins.
Also, only SOME of the 600 Strads in existence sound good. Many of them sound so bad that professional violinists won't play them! Major modifications that affect the sound have been made to almost every Strad. Additionally, the arching on the top of every old violin collapses to some extent because of the pressure of the bridge. The height and width of the arch has a major effect on the sound. It is believed that no Strad sounds like it did when Stradivari made it.
Briefly, what is so special about Strad violins is that dealers, who stand to make large amounts of money from doing so, have created the false belief that old violins are better than new ones.
By the way, the man's name was Antonio Stradivari. When he worked, ca 1600, Latin had an aura of sophistication, so he latinized his name to "Stradivarius" on the labels he put inside the instruments. Today, by convention, "Stradivari" refers to the man and "Stradivarius" refers to the instruments.