I see 3 major factors:
- Even more casual players reach 54 traits in at least one spec now or are very close to it (50 or 54 doesn't really matter that much). The motivation to do stuff for AP (solely or partially) is going away for more and more players. The prospect of having 19 traits "refunded" just to get back ~4-5 traits doesn't help.
- Thanks to M+ and WF/TF even semi-decent and semi-active players have average itemlevels of 885-900. The motivation to invest time and/or effort to do (relatively) challenging content (heroic or mythic raids and grinding high M+) for relatively minor upgrades isn't as big as it was doing it for significantly better gear - with no other way to get it.
- For raids with mythic aspirations the pacing of Legion has been terrible. EN was way too easy, whether it was intentional tuning or just the legendary/WF/TF/M+ systems, so a lot of raids not only stepped foot into it but progressed far or even cleared it pretty fast. Then came a very short-lived (especially because we had Christmas/new year during it), hard tuned ToV and NH was announced to come out pretty quickly which caused many raids not to invest too much into ToV.
And now we have NH with the 3 first bosses being absolute "free-loot" for every decent mythic raid. Since there is essentially no real progression through the first 3 bosses, a huge amount of raids that usually take weeks/months to really get into mythic, instantly stood before the significantly harder bosses and get their asses handed to them. Part of that is tuning, but part of is that players are not prepared to play harder boss encounters anymore. It's not like Krosus or Tichondrius are rocket science.