Mostly it isn't the sound guy based on the antics of primadonna (inexperienced) bands that I've seen.
We played a local pub 'festival' for several years, the last one the local 'famous' band had been relegated to support.
Speaking to the sound guy (line up organiser), he was less than complimentary about them, their lack of professionalism and level of expectation. He said he'd put us as headliners as we just "got on stage and played".
It was quite painful listening to band after band "soundcheck" throughout the day.
We should remember that the sound guy is often the person who will report back to the venue organiser and recommend whether a band comes back and plays a more popular night, or never returns. There are more bands than venues.