Banning is based on what is horrible and detrimental to the meta and what can score a 4hko without ever getting hit about one in four times while being faster than all of OU and able to drop your SpDef by two stages nearly two in three times it uses its base 120 power STAB move off of the same base SpA.
EDIT: When you're making a good point, abnegation, don't ruin it by making a wrong claim that Terrakion is outsped by Excadrill at +2 spe.
My bad,took a look at my previous post where I calc'd Scarf Terrakion.
Originally Posted by Fat JT Swift

Excadrill makes scarfers obsolete? Really? What if I need different resistances to switch into something for the revenge kill? What if I need to use a different type of attack? What if I need to use a special move? Scarf terrakion still outspeeds everything regardless of excadrill's existance.
Scarf Terrakion - 519 Speed
Excadrill in the Sand - 604
So no Scarf Terrakion does not outspeed.