The timed nature of Mythic dungeons is not for everyone. It creates an artifical difficulty and it can make you feel rushed. Thats not everyones cup of tea.
Furthermore, if you remove the AP grind, they lose quite some of their appeal. Sure, you get ear, but getting the right WF/TF gear doesn't feel that rewarding. Its more feeling relief when the piece finally drops. If they weren#t tied so deeply into the crappy legendary, WF/TF and AP system they' d lose a lot appeal. So the question is whats next? I really hope they get rid of the AP grind and still manage to keep M+ interesting going into the next addon.
Another thing is that they aren't balanced. HoV should reward 1/3/5 chests to keep it fair with shorter dungeons like MoS (same as BRH), while DHT and EoA should be 1/2/4 to account for their length.
another thing is the community. You can do perfetcly fine up to 7 with only 850-855, and 865 for everything up until and including 10. But most groups have ridiculous requirements, which makes in daunting for players who "fell behind" to get back at it again. While I alaways advocate to find a guild that has similar interests and getting into M+ that way, its still something that needs to be kept in mind when asking why some people don#t participate in M+. You can#t get the ACM for +15 in time when you only get into an M+15 grp if you already have said ACM.
Source : http://www.mmo-champion.com/threads/2169358-Why-do-you-not-like-Mythic-dungeons