I'm going to take a guess and wager it's because, at the time, the elbst was not a flying mount. And, far as I know, they haven't released a non-flying mount in a long time. So, no sense in releasing a mount that couldn't fly, and not worth the trouble of making that elbst fly, followed by the outcry to make the beast tribe mount also fly.

The elbst was one of the last mounts to be granted flight, so maybe there's a chance of it now, if they ever come up again.