It’s not that they will be “similar”, but from what i’ve seen is that there is no clearly define design for tanks.
Keep in mind that all tanks in some way had a unique identity in their playstyle:
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Warrior is the traditional sword and board tank vs physical damage
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Paladin is/was the aoe tank and closest to share a design space with war (my opinion is that he suffers from being a really old tank along with warrior).
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Druid is the heavy armor/dodge/hp stacking bear
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DK is the self healing one from damage taken
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Monk is the one that staggers dmg
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DH is the one that heals from damage dealt
As you see, apart from the original 2 (or maybe 3), each one has a clear design along with theme.
The problem with shaman is not that there is no theme, it is quite thematic, but the problem is that there is no clear idea for a design goal.
Asking for a shaman tank is going to a car manufacturer and asking for a red car. Sure, it is what you want, but it is not something he can deliver on that alone, since he must come up with all other specifications.
This is why I believe we got shaman tanks in SoD but not in retail. In SoD where specs are not really well designed, they can add 2-3 abilities and call it a day because it is thematic.
In retail they must build a well designed spec with its abilities and how they interact with everything in the game (and that they are unique enough and not share the design of another tank). That’s what I believe holds them back from delivering, not the lack of wanting it.