I agree with Bill Faust's theory of the demise of the cornet.
Last September at the Summit Jazz Festival in Denver I asked Doc Cheatham
why there was a shift away from the cornet and toward the trumpet about
1930. He said Louis
Armstrong was the reason. Doc claims that people now don't have any real idea how popular Louis was. Louis could pack the largest theater in Chicago every night for a 16 week gig.
Doc said that Louis thought that the cornet was an OK instrument but it really wasn't sexy enough for jazz. Everyone wanted to sound and play like Louis, so when he switched to trumpet about 1929 everybody else switched, including Doc Cheatham.
Now you rarely see a cornet outside of a trad jazz band. There are a few cornetist out there like Warren Vache and Ruby Braff. If you don't know these guys, you should. They are absolute wizards on their horns.