Like most Magic: The Gathering cards, the value of Mabel, Heir to Cragflame depends on a few consistent factors: its rarity, the printing and finish (regular or foil), its physical condition, and the set it was printed in (Bloomburrow)
It costs five colorless mana to use and has an ability that allows the player to tap any number of Creatures within the player's control with a total power of 2 or more
More so than most of the other cards on this list it lets you outplay your opponent as well which is always an indication of good card design
Mirrorwing is significantly better than the terrible Mind's Dilation, but still doesn't make it in Modern
Most magic decks are highly focused one or at most two strategies for winning
This is a non-relevant body in EDH which makes him appeal a bit more than Eternal Witness and the ability to get a card, sac him to Greater Good or something and get another one and other shenanigans makes him really appeal