The visitors were denied an equaliser in first half stoppage time only for Jay Simpson's close range finish to be ruled out after Stephen Quinn was adjudged to have handled in the build up.
Bruce said: "I thought Cardiff were the better side and we were nowhere near the level we've been at in recent weeks.
"But it might have been different had the disallowed goal been given.
"It was a big turning point in the game. It's a baffling and ridiculous decision and a big call.
"It upsets me greatly because I think the linesman flags for offside. He realises he has made a mistake and then when he was walking off at half-time he says it was handball which makes matters worse because the only person who handled the ball was (Cardiff defender Ben) Turner.
"The ball hit his arm - it didn't hit anyone else.
"It's a difficult one to take. Even though we haven't played well in the first half, we were getting into the game in the last 15 minutes.
"If we'd scored then it might have been a different story."