This is a good question. In truth I don’t know.
I personally like the story where the Titans come to Azeroth and find the Old Gods’ presences, defeat them, and then order the world at little bit, but leaving most of it as it. In either case however the question has to be asked why the Trolls and Tauren don’t have Titanic metal construct of themselves walking around. Why did the Titans either leave or make these races?
One possibilities is that the Trolls and Tauren are beast that were originally created on Azeroth, then were exposed to the Well of Eternity. However seems like a bit of a stretch, especially considering the Trolls and their connection to Elves.
Another theory is that the Trolls and Tauren were left alone and the Titans just created their own races, expecting their creations to wipe out the weaker fleshy race.
The last theory I can think of is that the Trolls and Tauren are the original constructs and there are no more metal versions of themselves. Possible because they wiped them out or they just all got the curse.
Sorry I can’t be of more help.