Because if harmon was out for the year (sounds like he isn't) when Ogunjobi returns he is plug and play having spent the last few seasons there.
also it sounds like they an abundance of corners and Bennie Bishop Jr. is not going to make the team after their free agent signings/trades.
Beanie Bishop Jr.
Pittsburgh Steelers · CB
Last year, Bishop was quite the find -- an undrafted free agent who played all 17 games, started six and plucked four INTs (including two vs. current teammate Aaron Rodgers). At worst, it appeared Bishop had a nice future in Pittsburgh. But the Steelers signed Darius Slay and Brandin Echols and traded for Jalen Ramsey this offseason, pushing Bishop to the margins. With multiple players already capable of playing the nickel, Bishop might be expendable. DB coach Teryl Austin recently said Bishop needed to "fight and work to get some splash to get in the picture." It wouldn't be stunning if the Steelers floated him in a trade.