They made a logical progression by the Forsaken being the humans of Lordaeron. Otherwise by your own logic, the Draenei should have joined the horde because they're tribal. You saying Night Elves are druidic not tribal as if that doesn't also describe Tauren is a contradiction in your logic.
You also seem to be forgetting the High Elves turning on the alliance was also a major storyline of Warcraft 3