Because NB is more important than Outside CB in this scheme. Your outside CBs will typically have S help deep and will play outside leverage because the goal is to funnel a WR back to the middle of the field where there are other defenders. Your NB needs to be stout in the run game and more physical in the pass game as the WR can break in, out or down the seam where they can split the deep safeties. A lot of teams will move their best WR around and line him up in the slot. Packers did it with Adams last year for example.
Cruishank would likely play dimeback where Gordon still covers the slot and Cruishank covers the TE. If he were NB then he would not be covering the TE but the slot WR. So he likely replaces Morrow with Quan as the only LB in obvious passing situations where there is still a TE on the field. That or he could play nickelback in situations where they line the TE in the slot while still having 2 TEs on the field.
As NFL passing attacks have evolved, so too, has the modern nickel back position. Hall of Fame coach Bill Parcells breaks down that evolution
www.the33rdteam.com
This is Bill Parcells explaining this. That is HoFer Bill Parcells. He also mentions how he spoke to Bellichek and Zimmer on this and points out that Ramsey moves to nickel as he plays what they call the Star role. So Gordon is being asked to play nickel precisely because he is one of our best CBs and best suited for the role as he has quickness, speed and strength. The modern nickel back basically needs a combination of CB and LB skills.
