The reason is mainly “the lore” as I understand it.
It is just that shamanism can “exist” within cultures that exhibit some characteristics. In the alliance side mainly draenei (that have turned to the elements when they were abandoned by the light), dwarfs that retained a stronger connection to the earth from other titan created races, pandaren that were supposed to be more “in-tune” with the land of pandaria and finally Kul tiran, a culture that with their connection to the sea, turned to a form of shamanism.
From what I can understand for the other races:
Night Elves: After evolving from the trolls and building a culture around the nightwell, they view the elements more like an arcane subject to be studied, and not like being to connect (like all shamans do).
Gnomes: Same as Night elves. Although being close to their cousins the dwarves, their curiosity by nature, has guided them to research/explain and advance their knowledge. They do not view the elements as beings to trade with, but only to be used as resource.
Humans: Of all the alliance races, humans are the one that “could” be close to shamanism. My reasoning as to why that did not happen, is that we play strictly as “stormwind humans” that turned to the light instead of the elements at the dawn of their culture. I could see human settlements that evolved otherwise and their culture was centered around shamanism rather than the light (think of tribes like the vrykuls).
Worgen: For all intents and purposes, humans cursed by druidic curse. More or less what applies to humans apply to worgen.
Lightforge: They never lost connection to the light, so they never needed/turned to the elements.
Void Elves: Mostly void touched blood elves. They had a culture akin to the humans and night elves. Never needed to turn to shamanism.
That seems to be what is available in the “world” of warcraft. On the other hand there are rare cases that we have members of a race being any class they want (like a forsaken paladin), but that is mostly attributed to rare persons that strayed from their culture.
I am in favor of playing any class with any race, since we are playing as a “mawalker” or whatever other special snowflake we are. I find little reason to not let player characters choose from any path available.
If I may, I think that dwarf shamans make sense not because of all dwarves maintaining strong connection to the earth, but rather the Wildhammer clan in general. Wildhammers have a style of living that’s very similar to the orcs imo, especially orcs before Ner’zhul got tricked by Kil’Jaeden. Dark Irons only make sense as Dark Shamans tbh.
Also, playing as any class with any race wouldn’t work well at all. Imagine what’s going to happen on RP servers for example. I personally don’t want to see a Dranei, Lightforged Draenei or Mag’Har Warlock - since being a warlock would turn them into other beings, they could literally not exist. Same goes for any other type of Demon Hunter besides what we have now - it would be horrible seeing them everyday - or Undead Paladins (Undeads are hurt by the Light, Priest makes sense because of Shadow
You are mostly right about the Wildhammers, and the Bronzebeards had a culture that forsake much of shamanism. The only part that I disagree is:
Dark shamans were individuals that enslaved elements instead of making bargains with them. While I believe at the start they tried to get the power of ragnaros, at the end THEY were enslaved by them. So I do not know if the “dark shaman” term could be applied to them, since they actually had a relationship of “server” to the fire elementals.
As for the all races/classes combination part, I think it could work in the lore, as they already have it (yup, even forsaken paladin), but I do not feel so “strongly” about it.
