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It was because it was the decades immediately following two things: Maxwell's discovery of the classical theory of electromagnetism, and the maturation of abstract mathematics in the 1800s (complex analysis, topology, differential geometry, linear algebra, symmetry groups, curved spaces/calculus on n dimensional manifolds, etc).

Electromagnetism was THE big "opening of the flood gates" for modern physics to arise out of the post-enlightenment, classical/Newtonian physics that was finally on the right path thanks to Galileo and Newton, but still not "mature" and largely done by wealthy gentleman science with imperfect and inconsistent methods and often sloppy mathematics/bizarre notations, dense and cryptic formulations.

A lot of people don't know this, but once Maxwell had cracked electromagnetism, special relativity was imminently inevitable. If Einstein hadn't been the one, one of his direct peers, Plank probably, would have published something roughly equivalent to special relativity and been the ones themselves. Like within a year or two of Einstein's publication date. Possibly Maxwell himself if he hadn't died early of stomach cancer, is fun to think about. Everything at the cutting edge of physics in the late 1800s, very early 1900s was pointed right directly in the direction of special relativity.

Then the development/maturation of abstract mathematics made it so electromagnetism also led right to quantum mechanics with minimal delay.

It's really general relativity that is the major profound "holy fuck" brilliant individual insight physics revolutionary achievement that could very well have been undiscovered for a while if Einstein hadn't personally done it. Special relativity and quantum mechanics, comparatively, both tumbled right out of the theory of electromagnetism after poking it a bit plus the coming of age of math.