Honestly? Did you remove the tundra mammoth? The yack? No? So why remove the brutosaur?
You’re obviously trying to push mass token purchases, since this announcement coincides with the increase in number of tokens which can be purchased per account per year. Why not just stick a $550 price tag on it in the store? That’s essentially what you’re doing by telling people they need to farm it now or it’s gone in 2020.
There were not problems, with the huge size of the mounts you mention.
They probably realized their mistake, as more people get more of them. However, they can’t just “remove” what people already bought. Nor can they really nerf its size, or make it have a short neck or something; People would freak! So they probably figured they should just limit it more.
They are probably adding a more sensibly sized AH mount to Shadowlands, but don’t feel like spoiling it yet.
They think it looks better to have a stealth $550 mount than an explicit one that gaming journalists and YouTubers can craft scathing reviews and criticisms of.