Unlike McGonagall said, the articles about the truth of their parentage don't make much noise. The prophet is too busy telling the story of Neville Longbottom—the  Real Boy-Who-Lived—to care about the fact that Harry and Draco are technically bastards. People don't make the connection instantly, the two teens too different looking to be confused—cer, correctly called brothers. The fact that people all over the wizarding world are trying to suck up to Sirius and not get on his bad side as he reclaims the Lord Black title also helps.