I don’t know the “Why” but blizz is trying to kill Shadow priest. They are dumping everything into Disc priest being the best healer and abandoning shadow completely. They do give some attention to holy but it doesn’t seem like they have any goal to make it competitive with Disc based on rankings, outputs, community opinion.
When S2 started the Spriest was “meh” competitive: no required buffs or utility, competitively ‘mid’ in Damage output, bottom tier survivability (arguably dead last on this), and definitely dead last on movement. There was however one situation where Spriest was a top 6 (6th place) and that was Single target boss fights with no movement - There was one boss this applied to “One Armed Bandit”.
Blizz was very against Spriest being anything but dead last so they nerfed all damage 3%-5%. The patch notes said 3% but people have been doing some diving to find out that some abilities were nerfed more so. Because of our design and how we scale this ended up being a 9-12% total dps reduction in M+ overall damage which really killed the M+ scene for the class.
Basically you can take a DK, Mage or Boomkin and get the 4M overall damage with a battle rez or lust and a true passive buff like MotW or Arc Int. Or you can take a Spriest who will be doing about 1.5M less on average compared to any top tier, and is going to bring 0 additional utility, is more likely to die, cannot keep up with the pack to pack movement requirements like other classes.
Purely for best possible if you look at any M+ dungeon you can see that the overall bests compared to Spriest bests are some of the worst DPS disparities the game has ever seen
The Below are all based on overall
Cinderbrew:
best = 5.4M
Spriest Best = 4.1M
DFC:
Best = 4.2M
Spriest Best = 2.9M
Flood:
Best = 5.6M
Spriest Best = 3.8M
Workshop:
Best = 3.6M
Spriest Best = 2.7M
Priory:
Best = 5.3M
Spriest Best = 3.9M
MotherLode:
Best = 4.8M
Spriest Best = 3.3M
Rookery:
Best = 4.9M
Spriest Best = 3.4M
ToP:
Best = 3.6M
SPriest Best = 2.6M
Given that “The Meta” and perception are the two most important things to a season and what classes make the cut there is just no reason to bring a Spriest to any content. A Spriest could get through the dungeon and time runs but why bother hoping the Spriest is going to be a top 1% player of the class when you can bring any Unholy DK and they can out dps them without even trying.
The same basic disparity is present in most Raid bosses; the only notable callouts are Stix Bunkjunker and One armed bandit where Spriest still falls into the middle of the pack and is somewhere between 12-18 rather than bottom 3.
Overall there are some fundamental changes this patch which were all detrimental to the Spriest and their kit.
Big impact M+ changes:
- Only Interrupts stop spell casting, Only silence and with it requiring 2 talent points for a 30s CD silence Shadow priests need to give up no less than 3% of their total damage to have a competitive and helpful interrupt. Psychic scream can no longer truly stop spell casting. The mobs will just immediately cast it again.
- Higher mobility requirements in all new fights. Very few fights truly permit a DPS the ability to stand; other classes with movement issues saw updates and changes to fix this while priest saw no such address. Voidweaver offers moving while using torrent but is a much weaker option compared to Archon. It is literally worth more dps to lose a GCD in archon than to go voidweaver.
- lots of High quantity non elite mobs. Pretty much every dungeon in S1 and S2 (and all TWW new dungeons) have 2-3 areas where the trash pull is more than double the total target cap of Shadow crash. Only Spriest is negatively impacted by this and every other DPS class has a viable option to unload damage on these packs.
- Very bad gear itemization; Shadow priests are very dependent on Mastery and Haste and most gear is poorly itemized with all of the best non set piece items coming from raid. This vastly limits options and means “luck of drops” plays a much larger role in you being able to be bottom dps or bottom dps but better than other shadow priests.