Variety of reasons, with some geographic thrown in. Lot of parts in the 1911, relatively complex. Revolvers have been reliable along time now, and with moon clips and speed loaders reloads are on par with mags with training. I know in Maine a lot of cops were loathe to give up the .357mag. It's nice having a round that handles things bigger than people if the need arises. Being able to chamber it with the .38 loads you had for a back up J frame was practical. The 9mm enjoys a good reputation now thanks to modern bullet design and better powder, but that wasn't always the case. I think people could see semi-auto was the way of the future, but they were reluctant to leave their tried and true problem solvers for small rounds. Enter 10mm/.40S&W, .357SIG, and .45GAP development.