Why do cyclocross bikes use 1x drivetrains?

1x drivetrains dominate CX for three reasons: reliability, simplicity, and mud clearance. Without a front derailleur, there's one fewer component to foul with mud, one fewer mechanism to drop chains under the jarring impacts of off-road racing, and one fewer cable to maintain. Clutch rear derailleurs keep chain tension secure over rough terrain. The gear range of a 1×12 setup (e.g., 40T chainring with 10-33T cassette) is adequate for the short, punchy nature of CX courses. For riders who also ride significant road miles, 2x remains a viable option.