Dragons can canonically transform into anything.
Dracthyr are historically unpopular, and of the players who do play as them, most stay in their visage form. This just adds to the overpopulation of humans and blood elves.
Why can’t their visage just feature a full character creation screen and allow them to choose any playable race?
Oh trust me I’d love an Orc visage form instead of ugly as sin elves.
Although I like using male blood elves for a few alts, I find the visage forms of both male blood elves and human females to be hideous. I don’t know if I’d like other races any better, but I still find it odd that male blood elves and female humans were the only two options when making visage forms.
Maybe they’ll expand it to new races and give us more barbershop armor options someday…
Because it would require more development time.
The most likely answer…they only prepared the Drakthyr visage appeance customizations for 2 races
While the base bodies are there, they would still have to add horns, facial scales, the 2-color hair combinations, etc
And they will get pushback either way
“Here’s a race you can play that can look like any race you can play!”.
Just not Talon-ted enough 
Would be cool though if we could. I would have a visage form of the Vulpera.