The first half of the 20th century had a lot of "low hanging fruit" in physics and chemistry that basically came out of the enlightenment. Things like electromagnetism, quantum mechanics. I wouldn't guess that's always the steady state for research.

The war may have had something to do with it too, I know a lot of advances (like radio / microwave engineering) have roots in the war efforts. But I suspect it's more of just a dry spell because we pushed forward in the 18th-20th century and ran out of ideas