Around 1946,'47, and '48 there was a dramatic drop-off in triples. Why?

It doesn't seem to be connected with WWII. There weren't a lot of new stadiums built. There were no significant rule changes. Why all of the sudden did triples fall off so dramatically. Players like Stan Musial would hit 14-18 triples a year, then BAM! Leaders were sometimes not even in double figures. Willie Mays hit 20 one year but by then that was the aberration, not the norm.