Holiday mounts require level 98 to drop
They scaled the holiday dungeons so that everyone could do them, but you must be level 98 to receive the loot.
https://us.battle.net/forums/en/wow/topic/20761606235
Quote:
As a compromise, we made account-wide rewards a random drop for any character capable of participating in current-expansion content. For the remainder of Legion, players level 98 and above fit that criteria. The minimum level requirement will increase again in Battle for Azeroth, and will continue to increase in future expansions.
Since its a random drop, many people may have been farming on low levels thinking they were just unlucky.
