A bit of engineering and a lot of myth and degradation due to time.

The engineering seems to be a combination of genuine construction advances and the usage of wood that was abnormally dense due to having been grown during a big drought.

This, of course, contributed to the "myths" around the Strads with the varnishes, techniques, etc. supposedly being "The Thing(tm)" that made Strads so much better.

Finally, wood degrades with time--period. It doesn't matter how much you try to preserve it, it's just fact. The current Strads are either "Ship of Theseus" type violins, or they are heavily degraded.

At this point, modern luthiers create better instruments than even a Strad in its prime. They have access to better woods, better glues and finishes, better tools and training, better analyzers and better players than anyone in the time of Stradivarius.

When played as close to double blind as is possible, the data comes back with modern players preferring modern violins made by modern luthiers over the old Strads.