Don't know where "most came from.. Even in Los Angeles there are very few AM HD stations, and far less than 50% on FM, and those numbers go straight down as you leave town.
HD is not static or noise tolerant AT ALL. It takes nothing for any of the FM-HD channels to drop back to analog, including the grandfathered 100KW KRTH.
If you are in rock solid coverage of the station, then yes it's wonderful, but any hill, noisy car, including allot of the electrics, power lines, AM Skip, or phase distortion on either band will drop any HD service right back to analog.
Personally I get tired of the radio dropping in and out of HD service and usually shut it off