What's the main reason why people get pulled over when driving drunk ?

Not know why you want to know this information I must caution you about drinking and driving. Alcohol impairs the brain in so many ways that normal anything is not possible. That being said, there appears to be no main reason. Asking the police would give you the best possible answer. But depending how drunk a person is, impaired drivers forget to turn on headlights at night, they hit other cars, buildings, etc., swerve in/out traffic lanes, go too fast or too slow, do not stop for lights or stop signs. Drunk drivers kill about 16,000 people each year and cause thousands of injuries. It can land you in prison or jail and can cause permanent injuries. Do not drink and drive. OK?