Seems like both sides dropped the ball here. I never heard "Air Canada" on either call - only "178, pull up, go around", and no reason was given for the late call. Something like "Air Canada 178, go around, vehicle on the runway" might have gotten their attention. But the crew should still have been on top of the call - BECAUSE they were in such a critical phase of flight. On short final in Charleston, WV late one evening, I got "go around, herd of deer crossing the runway" - thankfully the tower controller spotted them before we met mid-field! You always land primed to go around at any moment...