Why was Aegislash banned from the OU tier of Smogon?

Smogon is a website that discusses and plans strategies for each Pokemon, and even creates a tier system based on usage and power of each Pokemon. This ban may not apply if you don't play under Smogon rules, but Aegislash has been banned to the Ubers tier, which is typically filled with legendaries and Pokemon such as Mega Blazekin or Mega Lucario- which are all very difficult to counter, let alone beat. But why Aegislash? Let's look at the Pokemon itself.

Aegislash can switch between Sword and Shield mode, with each mode having these stats-
Shield Stance: 60 / 50 / 150 / 50 / 150 / 60
Sword Stance: 60 / 150 / 50 / 150 / 50 / 60

This Pokemon has several different viable strategies, SubToxic, Physical, or Special.

To begin, his Physical set can be used with Swords Dance to boost his Attack stat by 2 stages in one turn, Shadow Sneak as a priority move to help with his lackluster speed, Shadow Claw for STAB/Sacred Sword for coverage/Iron Head for Fairy Coverage, and finally King's Shield. This is Aegislah's signature move which functions as Protect for offensive moves-which lowers the foes Attack stat if the move makes contact, however it allows status moves to bypass and hit.

For a Special set, Shadow Ball is a very good move to have for STAB along with Flash Cannon, and use Shadow Sneak and King's Shield just like the last set.

You can use the item Weakness Policy for both sets for a +2 stat boost in both Special and Physical Attack stats after being hit for Super Effective damage. With low speed, Aegislash will remain in Shield mode and keep its defensive stats.

If you want to try something else, the SubToxic set is also a good choice. Substitute, Toxic, King's Shield and Shadow Sneak will make Aegislash an effective staller especially with Leftovers.

As you can see with all of the various sets, Aegislash can pull off many different strategies and with such high base stats it is easy to see why it is used in so many teams and was sent to Ubers and used on so many VGC championship teams.