I think with the influx of people moving into urban zones, you get a lot of the suburban "biking is for hippies, so who cares?" mentality. I moved into a city five years ago, and while I quickly learned about dooring both as a driver and cyclist, I can't help but wonder how many people just don't realize that it's "a thing". Couple that with the hatred I often see towards cyclists, and I can pretty easily see how people either don't know or don't care to look before opening a car door.