There is a site that tracks how many characters own each mount.

Sleipnir - 23%
Fat Moogle - 22%
Fatter Cat - 22%
Aquamarine Carbuncle - 19%

For arguments sake, I'll use Lucky Bancho's recent census. I will also assume out the 1.1M active characters, only a quarter of them are individual players. At $24 each, this amounts to $26.4M reasons why SE changes what they do. If cutting that in half, it's still a staggering amount of money. And that's for only four mounts.