User blog comment:Danz The Pure/10 thunderlords in one 5v5 ranked game/@comment-24420226-20160103202837

It's just the most generic, simplest to use keystone mastery usable on literally every single champion in the game. A lot of the other keystone masteries are more niche - not everyone builds crit, not everyone autotattacks rapidly, not everyone goes tanky. Deathfire touch is a little underopowered and has caveats for DOTs and AoEs. Plus, it's the keystone mastery out of the "utility tree", which itself is very flexible and has tools applicable to multiple champions and scenarios.