It's not rocket surgery, it's football.
Namely, it's pass coverage. Defensive backs, by and large, either cover an area of the field (known as a "zone") or they cover an eligible receiver as he runs a route, looking to achieve separation from said defensive back and catch the forward pass from the eligible thrower (known as a "prima donna" or a "quarterback").
The Steelers did not select one of these defensive backs - namely a cornerback - until the fifth round of the Player Selection Meeting (known as the "draft"). Why? Some combination of lack of opportunity to select the cornerback they wanted, or a lack of value for that position with that specific pick.
Reading between the lines, they feel they didn't need to address the position with an earlier pick, or at least, they didn't feel any cornerback available to them before the fifth round was better than their options.
Steelers defensive backs coach Carnell Lake broke it down simply, and generally, throwing his support over two players the team didn't draft, but signed in free agency in 2013 and 2014, respectively, Antwon Blake and Brice McCain.
"I think these guys are just getting started," Lake said of Blake and McCain, as quoted by Ray Fittipaldo of the Post Gazette. "They have the ability to cover and stay close to receivers. So for guys we've picked up in free agency that may have been overlooked in the past, I'm pretty excited about them. I think we have the ability to plug and play some of these players we've picked up."
Imagine the innovation coming out of the Steelers' South Side headquarters. They don't need to draft a cornerback...because they already have cornerbacks.
The earth revolves around the sun, people.
It's fair to question that philosophy, like so much loaded hyperbole and unintentional arrogance. If Blake and McCain are so good, why did Jacksonville and Houston, respectively, discard them, these wastes of proverbial air?
If the challenge is so complicated players can't accomplish them, the conventional thought is to find players who can successfully overcome that challenge. The shrewd tactic is to work, instead, to shrink the size of that challenge. Like Lake said, they have the ability to cover and stay close to receivers. He didn't say "shut down the opponent's No. 1 receiver" or "be the Steelers' version of Richard Sherman."
It's the KISS method - Keep it Simple, Stupid. Cover by staying close to the receiver.
This franchise has drafted three cornerbacks in the first round since 1987. All three became solid starters and one is in the Pro Football Hall of Fame. While most teams would love to boast a track record of 1-for-3 when it comes to drafting Hall of Fame players at a position, the flip side of it is probably more realistic, especially considering the team drafted players the likes of Chad Scott and Deon Figures in later rounds several times since then, and many of them out-performed the fairly pedestrian career numbers of both of them.
To put it another way, how smart would the Steelers be considered if they happened to have signed the 2014 equivalents of Scott and Figures to veteran minimum deals? Pretty gall durned savvy, actually.
Maybe Lake is inflating the value of both Blake and McCain. Maybe he legitimately feels they can climb to levels higher than their current status of reclamation project subjects suggest.
Even if they don't reach the dizzying heights of the abilities of Figures and Scott, they are players with some experience, with some speed and, hopefully, with the desire to prove the doubters wrong.
Maybe it's as simple as that.