Aiyuk is a really good player, but not good enough to be worth giving up significant draft picks for when he wants a big contact next year. Especially for a team that is not a WR away from competing, at this for this upcoming season.
Aiyuk had a lot of his production on deep (15-20 yards) in routes last season, which are long developing routes that the 49ers were able to get him open on because of the threat of the run game and they had really good pass protection. Since the Steelers do not have the same, I don't think Aiyuk would put up the same numbers for the Steelers.