Warlocks have a guaranteed spot in almost any RBG group whereas shadow is mostly a “eh, there’s nothing better in queue” sort of pick. I’ve had to play disc far more than shadow just to do my rbg push 
That’s circumvented with exp/gear. I played shadow till around 2200. A lot of classes are in a “if they have exp and gear, invite them” spot right now. Easier to invite a lock or boomkin and expect them to spam corruption or starfall and do damage than it is to expect a spriest to be able to use their brains.
I’ve never enjoyed shadow more than I do now, but every group I get, if I get an invite, give me the impression they wish I was a boomkin, afflock, or disc. It’s weird for a spec to be so strong and fun to play yet unwanted.
Thus, I am leveling my druid so I can flagrun as well as being the most wanted ranged.
FOTM will always be a thing because of Blizzard’s balance… How do you think a lot of warlocks feel when trying to find arena teams?
Granted, were a monster end of BFA… so I still think they can sink to the bottom of the ocean.