Here's my take on this, which is a rather longwinded one, but you might gain some insight into the design process:
The Class designers are next to unreachable from the outside, they live in their own world to a certain degree.
(A statement that might more sense after reading the stuff down below)
First off, they are biased.
This is not just me saying this, but Ghostcrawler said the same thing (more or less) during a LoL AMA back in 2016 regarding Frost mages in Cata.
https://www.reddit.com/r/leagueofleg...kp6/?context=3
Obviously, things need to be seen in degrees and not just extremes, but you have the Guy that took the heat for bad class design for years saying that the people who have a word to say in class design are biased in favour of the class they play.
Now, futhermore, take this long blogpost from Xelnath, who was mostly responsible for MoP Warlocks, while most of blogpost is regarding how MoP warlock developed, the introduction holds some neat tidbits regarding Blizzards inner machinations.
https://xelnath.com/2018/05/15/the-m...ew-you-missed/
Guess how Monks performed for a good chunk of MoP?
They were bad and abilities like Storm,Earth and Fire were even added later on to make them work, Monk wasn't exactly good class until late MoP / WoD.
You can only guess how the guy responsible for (Elemental / Enhancement) does these days, probably overloaded with work, and if no one of those people above him (which aren't without bias as established above) give any space to design Elemental properly, you get that what we get every expansion - half finished shit.
Xelnath then proposed to take Warlock design (with GC's approval), then wanted to get help from the community on board, which is apparently not a wanted thing at Blizzard because they have a "Wall of Silence" policy.
You have think about the bolded sentence, that really speaks a lot of about Blizzard as far as their companies design philosophy is concerned.
Looking for a rational conclusion is simply not possible, classes are bad not because they *just* made a mistake, they are thrown under the bus with full intent.
Guess what those people that put Elemental into a "low priority" do when the question of buffs is brought up?
Most likely, "Thumbs down, just a bit, to keep them going...whatever" nothing that could threaten the status quo.
There either are no people on the class team that care about Dps Shaman, they're not in charge of that or simply aren't given enough room to create anything relevant.
Just look at what kind of joke Tremor totem is in PvE, you_cannot_tell me that is just an unintentional error, but on the other hand those people hand out 5% phys / magic debuffs to Monks and DH or greenlight a boss that makes two warlocks next to mandatory even on heroic.
Xelnath also wrote:
Guess how that turned out.