Without commenting on the overall trend's cause, your diversity hypothesis is bunk and suggests you are looking to making things fit a diversity-related narrative:
- there's (unsurprisingly) no significant diversity-word change from 1900 to 1940 but a very significant distance drop
- there's a big diversity-word change around ~1990 with no concomitant distance change
- there's (unsurprisingly) no significant diversity-word change from 1900 to 1940 but a very significant distance drop
- there's a big diversity-word change around ~1990 with no concomitant distance change