Rich Homie Quan -- Bails on BET Performance After Stage Demotion

Rich Homie Quan is main stage or no stage at all ... and that's why you didn't see him at Sunday night's BET Awards.

RHQ was supposed to be on the telecast, but according to our sources ... he showed up to rehearsal Saturday and was told by producers he'd be the headlining act for the red carpet pre-show ... which did not go over well with him.

We're told Quan was pissed, feeling totally disrespected.

Fans noticed his absence, with one tweeting, "I mean this a real nice put together BET Awards but um... 😐👀 where's Rich Homie Quan???"