Because it's a feature and not content. Content is stuff to do. A class is not something "you do". You do stuff with your class. You don't do your class. That's the main difference between content and a feature.
- - - Updated - - -
Not really though?
Legion still introduced Mythic+ and world quests. Content that is still in Dragonflight but has been pretty diminished and worn off over the years. Both are not as exciting as they were back in 2016. They are still there, but they are base game content that is seen as mandatory nowadays. Similar to daily quests which were brand new in 2007 but are base game content now already (and nobody would question that, right?).
- - - Updated - - -
I never thought about calling Covenant teleportation mechanics - or anything fluff-related to Covenants - content. At least I never said that is content. When I refer to Covenants I usually do so because of a) their questline and b) their skills, soulbinds etc. which altered your gameplay quite a lot.
"The possibility" doesn't equal that we will have that. Not saying it won't come, but they have given us nothing in the last weeks to support their announcement. I just find Dragonriding - based on what we know as of now - incredibly dull. But that's due to how they presented it. I'm not denying that it could be fun and a good addition to the game in the end. But we're barely there yet to draw that conclusion.
- - - Updated - - -
Yeah, it's Blizzcon. And yeah, I know Legion was announced at Gamescom.