For a while, 160NC was the only available color negative film. Then Kodak started requiring a minimum order of 30 boxes of 50 sheets each (about an $8,000 minimum order for 1,500 sheets). Then all 160NC was discontinued entirely. Then the new Portra 160 came out, and the price for a box or 5x7 went up about 25 percent ($65). So, it's yours at almost $6.50 per sheet to purchase (plus $5 per sheet to have processed) as soon as Kodak finally releases it.
Plus Fuji still makes their transparency films in 5x7 TMK. But they are limiting in many ways compared to negative film. They must be scanned or used to make expensive Ilfochromes, plus their ability to record wide ranges of brightness is nowhere near as strong.