The reason they removed it in the first place, is if you remember, they were having server stabilization issues with the mounts. The mount started out bigger than the current version now. Because of it’s huge size and the fact that multiple people would be using the AH on it, from multiple points around the game world at the same time, was giving the server issues, that they did not predict. It was basically causing lag for other people.
So at first, they reduced the size of the Bruto mount, which caused a lot of folks who had already purchased it to get angry. I think Blizz thought that would solve most of the issues, but it obviously did not. In the end, they decided to remove it altogether from the vendor. I would imagine their thinking was that if they left it on there, eventually a lot more people would purchase it over the next few years and thus cause even more stability issues on large servers.
By moving it to the BMAH, they could control how many were released into the gaming world. Say one every 4-5 months would be acceptable, versus a thousand more people buying them over a 2 year period if left on the vendor. HOWEVER, even that I think was too much as they mysteriously removed it from the BMAH for about 2 years. I think again, it was another effort to restrict this mount on servers. In their mind, this gave time for players who already had the mount to stop playing the game and reduce the number of these mounts, before they placed it back on the BMAH.
In summary: They made the mount too big and the AH vendor gave them unexpected issues, they did not foresee. As you know with this game, a lot of older players have stopped playing and left the game. Thus their mounts die with their inactive account. I really think this was their plan, when removing it from the BMAH for so long. Basically, they want to limit the number of these things on the servers for the future. I too left the game for about 2 years and recently returned. I noticed that a few of my old guildies that had the mount, no longer play the game anymore.
For this reason, I do not think they will ever allow it to be put back on a vendor for mass purchase. It is still a large mount, even with the reduced size they did back in the day. I’m not sure there is a mount larger, but I’ve been gone a long time, so I don’t know for 100%