Try to understand this. Blizzard is a business. Like any company they will invest in things they find have a good return on investment.
They have 20 years experience with these classes and they know which ones interest players, which ones are profitable and which ones are not. Obviously their marketing people have determined that shamans are not profitable.
I suspect that the problem is that Shamans are not popular in modern fantasy fiction. Look at the Lord of the Rings movies, there were Humans, Elves, Dwarves, Orcs and Wizzards (i.e. mages, warlocks). There were no shamans.
I think about the only time I’ve ever seen shamans on TV was on Northern Exposure and even there it only involved one minor character’s story line.