Someone asked about this last week and I have been meaning to post some of the things that I have learned from my reading & collecting activities and start a thread if there's interest. I welcome more insights because I know that I don't have all the facts straight myself.
Most experts agree that the cornet as we know it today began to appear
in France and England somewhere around 1815 -1825 when certain instrument
makers began to apply early valve configurations (piston, rotary etc.)
to circular "post" horns or hunting horns resulting in a fully chromatic
soprano brasswind with a mostly conical bore. Gustav Besson is credited
for advancing the cornet design during the mid-1800's and by 1875 the cornet
was *the* dominant solo instrument in popular music in both Europe and
the United States. As hard as it may be to imagine, cornet soloists
like Levy, Arbuckle, Clarke and the like were the late 1800's equivalent of Glenn Miller in the 40's, Elvis in the 50's and numerous pop/rock stars today.
Thus, between 1875 and about 1925 there were more cornets made (I'm told) in the U.S. and Europe than any other form kind of musical instrument (including pianos) but I don't have the facts to support this claim. And by 1900, the classic British/French short model shepherd's crook design (about 12-13" long), with interchangeable leadpipes and extra crooks for hi/lo pitch and multiple keys was the standard configuration and a virtual cash cow for most manufacturers and importers like Conn, Besson, Courtois, Boosey & Distin.
So what happened? Well, I think that the cornet's demise can be mostly attributed to a gradual shift in popular music tastes during the 1920's and 30's with community band music declining and jazz and swing on the rise. One obvious result of this shift was the rise of smaller jazz bands and big bands with more limited soprano brass sections requiring a type of horn that projected better and was more in synch with the new swinging sounds - the Trumpet. Trumpets had greater projection, a more brassy sounds and frankly could "rock the house" in a more exciting style than their warmer, darker and more mellow conical cousins.
So while Bix and Bobby Hackett still perferred the cornet, guys like Harry James and Bunny Berigan represented the *new sound* . And remember, the big bands played mostly to kids and somehwere in those crowds were the highly influential future brass musicians, most of which decided to follow Harry's lead and play the trumpet. Thus, manufacturers simply followed demand.
If you look at cornet design after 1900 you start to see cornets getting longer, less conical, losing the shepherd's crook (ie. the King Master models) and essentially becoming more "trumpet like". Especially in the 1920's you can see the "trumpetization" taking place with examples like the Conn Victor and Harry B. Jay's Columbia model that even featured an interchangeable trumpet/cornet leadpipe system. By the mid 1930's the classic cornet design was almost forgotten (except maybe in the brass band scene). The epitome of this might be the Conn 40 Vocabell model that was offered in an *A* version with a cornet leadpipe and a *B* version with a trumpet leadpipe. Other than that, they were identical long thin peashooter trumpet configurations.
Any by the 1940's the cornet was becoming an anomaly instead of the mainstream instrument that it had been.
So that's my theory - any others?