And that reason is yet unknown to them. However some conversation can be found on the beta between Cindrethresh and Azurathel, the Dracthyr representatives from the Horde and Alliance. They talk about it a little bit.
Cindrethresh: Zury! I wondered if I’d recognize you in your visage. But somehow I just… knew it was you.
Azurathel: I felt the same sensation. Your visage suits you well, my friend.
Cindrethresh: It does, doesn’t it? Ebyssian said to look inside and let the magic within express how I wanted to be seen.
Azurathel: His visage is that of a… tauren, correct? And it seems many dragons take the form of elves. I confess I am still learning the difference between them.
Cindrethresh: I guess the old dragon was right after all.
Cindrethresh: I hear some dragons even choose to look like gnomes, whatever that is.
Azurathel: Wrathion could not tell me why our visages are so distinctive. Perhaps ours were meant for a different purpose.
Cindrethresh: All I know is that I like what I see. In myself, and in you.
Azurathel: Well said, Cindre.
I hope they don’t just never talk about it again and we never find out. It’s already weird to have them limited in this way, but it’d be worse if they never explained it, and just left it as “Neltharion works in mysterious ways”.
There’s additional dialogue between them on this WoWhead article. Azurathel seems to be the serious one, while Cindrethresh is a bit more happy-go-lucky