I assume it was simply because they were there first when the Well came into being - they were drawn to it by proximity, and apparently it didn't possess a powerful "beckoning" quality that would effect the other tribes on the united supercontinent of Kalimdor. Azerite seems to be following a similar pattern, with no one initially being aware of it except for Gallywix, and that solely because he had Goblins in the area already.
"We're more of the love, blood, and rhetoric school. Well, we can do you blood and love without the rhetoric, and we can do you blood and rhetoric without the love, and we can do you all three concurrent or consecutive. But we can't give you love and rhetoric without the blood. Blood is compulsory. They're all blood, you see." ― Tom Stoppard, Rosencrantz and Guildenstern are Dead
There's Vampire Elves/san'layn
There's Half Elves kind of like Alleria and Turalyon's Kid and Vereesa's Kids.
Demonic Elves/Felblood Blood Elves and the Felborne
Spider Elves/fal'dorei
and you also forgot to list down the Void Elves. Which they are kind of thing now.