I just got off a chat with Apple support and was told the Nike + iPod app was removed because the iPhone 6 has the native hardware to track speed, distance, etc. The sensor is now obsolete I was told. I explained that I (and many others) used the app together with the Nike running site to track my workouts. It also has all my history. She was sympathetic, and suggested I send feedback to the developers and request that they bring it back. (yeah that'll happen)
It seems Apple is trying to wrangle all health tracking and send it through their new health app. The more control they have here, the more leverage they'll have to sell Apple watches next year. It wouldn't surprise me to see fewer and fewer 3rd party health partnerships with Apple.