I’ll tell you why.
He had access to good tonewoods, was talented, competitive, and he and his colleagues were working at a time and in an area where the best instruments fetched a comfy price.
What more explanation do you need?
Also, it’s been several hundred years. Simply put, by now, his clunkers have gone into the fireplace. Only his best remain. And they are only ever played by the best players. Believe me, Perlman can make any violin sounds its best. And the descriptions of the things by their players is revealing; things that would be considered temperamental flaws in a modern instrument are talked about as if they are high marks of quality. The picky oversensitivity that Jacqueline DuPre described in her Strad cello makes it sound like a Formula racecar, ready to surge forward with the slightest touch on the accelerator. In a modern student cello, that would be called a flaw. “This thing is uncontrollable, persnickety, and unreliable!”
And to be honest, in blind tests, audiences and even players are happy to judge old instruments and new ones pretty evenly. The best Zygmuntowicz and the best Strad are pretty much on an equal footing. Another way to put it is that the Strads aren’t always the best ones around. Perhaps the reason we can’t prove why they are the best is because we can’t even prove that they are the best.
Part of their appeal is their historical romance. If it’s not quantifiable, it can’t be proven by quantifiable means. And we don’t want it to be, not even luthiers. I’ve heard luthiers say that if they could manufacture a perfect violin every single time, they’d love to do so, but they lie. They like the hit-or-miss nature; I recall Carleen Hutchins laughing over that. With her system of free-plate tuning and dependence on electronics and physics, she could turn out some of the best instruments in the world reliably. The old-school luthiers hated her. She used to chuckle over it. “I’m putting numbers on their mystique,” she said.
People want the mysticism. They don’t want proof, thus the Strad remains beyond proof.
And let’s face it: if science will allow you to make a perfect fiddle every time, it will allow the other guy do it too, won’t it? 🙂