Money!
If we try to reason a bit more, a new mount in the story was long overdue i had the feeling if we don't count the moonwhale in cause it's the fanfest mount.
Also if we give it through the old savage raids etc. people would have it in split seconds cause unsync. I actually would had liked it more in like pvp to make people finally play again more pvp. Together with some weapons and maybe a second pvp glamour for this expansion cause the pvp rewards we got this expansion are the real crime here.
Not to mention that they bring also the Noctis FFIV event back where you can get the car for some mgp which is a 4 seater mount. So it's not like you don't get any mounts (and especially good ones with multiple seats) only from mogstation.
