Because it is a passive that reduces damage taken by 30% for a lot of the sustained pressure of any given class. A comprehensive list of things which get heavily mitigated would be (not limited to):
- dk diseases and autoattacks, deathcoils notcrit for 20k still get reduced a lot
- mage nether tempest, pet attacks
- warlock dots, like anything affliction does, most attacks from demonology (imp bolts, hits from the felguard/wraithguard), immolation ticks, fel flames.
- warrior deep wounds and autoattacks, overpowers
- enhancement shamans autoattacks, flame dot, many noncrit attacks
- hunter most attacks except a select few during cooldowns
- rogue autoattacks, poisons, bleeds
- feral druid bleeds, autoattacks, many noncrit attacks outside of cooldowns
- elemental shamans flame shocks, and generally almost all damage outside of cooldowns
- shadow priest dots, mindflay, shadowfiend/mindbender attacks, noncrit mindblasts outside of cooldowns
- insert more attacks that generally don't excede 40k damage here.
There you go, you gain a 10%-to-30% mitigation against all that stuff. This being on top of the mitigation you already get from your armors. And as Korzul said, it also requires no input on the mage's behalf. Flameglow is a premanent defensive stance which turns out to be better most of the game then defensive stance is atm and has no drawback except for the opportunity cost of losing the ice barrier.