This is because when they would lay a sheet on the ground and shake the branches of a tree, they would often find more species of beetles than anything else. However, they may have been extrapolating a bit far from the few places they tried this.

No, it’s because beetles have the most diversity of any animal order.

“The Coleoptera, with about 400,000 described species, is the largest of all orders, constituting almost 40% of described arthropods and 25% of all known animal species…”

First paragraph of the wiki article! https://en.wikipedia.org/wiki/Beetle