Because it's too hard to maintain a 20 man, inflexible roster. Flex raiding was too much of a game-changer. Mythic sucks in comparison, because you have to have people sit constantly, or risk not having enough people when someone inevitably doesn't show up.
How do you hold on to skilled people while being forced to sit them?
If mythic were 10 man, preferably flexible, then I would raid mythic. As it stands, it's too difficult to organize.
Also, the early difficulty of mythic is tuned for world first, so going in there when you've cleared heroic was too big of a difficulty spike imo. I'm in the minority, I know, but I liked the difficulty of EN. ToV was like hitting a brick wall in comparison, whereas I actually managed to clear EN with friends.
I don't like how mythic basically requires you to ditch anyone who isn't perfect. I would have had to leave a lot of friends behind to raid mythic in it's state during ToV, and I'm not sure it's much better now.
Maybe mythic just isn't for people who want to raid with friends. I don't really have a huge issue with that, but I prefer when you can (soft) carry a few people, and one mistake doesn't wipe the group.
Tl;Dr: I just think the mode is tuned for the top 1000 players, at least early on, and requiring 500+ attempts on stuff like KJ is just not fun. That's forgetting that maintaining a strict 20 roster is such a pain. If these are irreconcilable facets of the mythic experience, then it's just not for me.