So what I began wondering this morning is why. Many disproportionately specious groups have powered flight (the insects when compared to other arthropods are much more specious, the same is true of birds and bats when compared to their closest relatives) but powered flight is a shared characteristic of beetles and other insect groups. So I went searching and found a partial answer on the website of UC Berkley's Museum of Paleontology which focuses on teaching evolution.
It seems that more beetles, compared to their closest relatives, feed on angiosperms (flowering plants). The theory goes that when angiosperms first began evolving more beetles than other insect groups began to use these new organisms as food. When you compare lineages of beetles that feed on angiosperms to their closest relatives that do not you see a clear pattern: the families that feed on angiosperms are much more specious. But again...why?
The answers are still not clear but scientist have put forth at least one good theory. Basically the idea is that the angiosperms are not providing a single new niche but a whole host of them by containing a number of new food sources: root, seed, leaf, fruit, etc. By being the first insects to "colonize" these new niches they were able to undergo an "adaptive radiation" across all available food sources. The theory is similar to the rapid speciation of cichlid fish whose mouth parts evolved rapidly to take advantage of available food in what might otherwise have been a single niche.
Evans, Arthur V. and Charles L. Bellany (1996). An Inordinate Fondness for Beetles. Henry Holt and Company Inc. NY, NY.
Understanding Evolution: http://evolution.berkeley.edu/