Hello,
The tiger for humans and zorb for magmars were one of the first mounts of the game (if not the first) and level restrictions were set depending of the strength of players at this moment. Now players have access to better equipments and stuff more easily but lvl 6 tiger/zorb is still a good mount for lvl 5-6.
By the way mounts lvl 10 are accessible quite easily for players from lvl 6 like endagar, shankar and grumvol if you work on the corresponding red medal (Brotherhood of virtue, Bringers of evil and Underground knights).
Reputations are an important part of the game if you want to improve your game play and those 3 level 10 mounts are, in my opinion, stronger than lvl 10 tiger/zorb and this is why we barely see players using it in fight.
So if you want to be rewarded with a better mount at lvl 6, you may work on your reputations or wait for lvl 9