The Browns have had the most number 1 picks in the last 10 years and haven't exactly hit on all of them. Giving up a 1 for a proven Pro Bowler in his prime is money.
My guess would be that the Steelers didn't want AB to go to a division rival. Browns one upped them by getting OBJ instead. That trio of Mayfield, Landry, and OBJ will be lethal! And Njoku at TE.