The reason: BYOD
Vue/Sling/DirecTVNOW all functionally enable a customer to bring their own hardware. with 3 TV's in my house, almost half of my cable/internet bill is spent getting the FiOS media server with the extra streaming boxes to allow cable TV to all 3 sets. With Vue, I can use my own Playstation/Roku/Fire TV to control it, and dont have to keep paying 50+ a month in equipment rental fees. If PS Vue gets Viacom channels back, I'm in all likelyhood dropping FiOS cable and switching.