Probably to match with the other players.
Pop's NY Times onituary sez:
Moving back to Chicago in 1925, Mr. Armstrong again played at the Dreamland Cafe, where his wife, Lil, had her own band, and with Erskine Tate's "symphonic jazz" orchestra at the Vendome Theater. It was at that point that he gave up the cornet for the trumpet.
http://www.nytimes.com/books/97/08/0...rong-obit.html