Title covers the basics. Currently all raids that aren't the newest raid have the Master difficulty disabled unless they are the featured raid. The Root of Nightmare seal can still be claimed on the Bungie Store through the end of the year but there will only be one more featured raid week for it before the deadline. Vow, VoG, King's Fall, and RoN all have desirable adept weapons that can only be acquired once every 7 weeks. The nature of RoN having fixed weekly drops means that you can only get one Adept weapon from the raid every 7 weeks. Not enabling Master for these raids simply artificially gates loot.