The data isn't mine but what I found on a quick google search. Hence why I linked to the discussion reddit. I suspect they used the Mandragora because it's the cheapest mount available but someone else provided the data for the others.
Garlond GL-II: 271,050 (23.8% of total active)
Island Alligator: 174,904 (15.4%)
Island Mandragora Queen: 111,182 (9.8%)
Island Buffalo: 85,139 (7.5%)
So the "actual popular mounts" weren't all that popular. Or rather, Island Sanctuary itself wasn't.
As for why they didn't Ozma or Elemental Gear +2 because it wouldn't be relevant to the overall participation of Eureka but rather Baldesion Arsenal. Neither of those things would matter to casual players, especially the +2 which is only a stat boost for BA.