If you compare the stats of the legendary air drop cards with the price after 2 weeks. You can see there is a correlation between price and score. Score calculation here.
Grum is pricier Lira for the same score. Mainly cause Grum has a good place in a fireteam with "unlimited" mana.
I believe there are already great off-tanks for the dragon group, Carnage Titan does not have the same value as Grum. And his stats are just underpowered compared to Grum.
Flip-tip:
Sell your air-drop monster on day one and buy it back with a discount! (or not, i don't like him anyway.)