Why is Faker a Sub?

Faker is the more dominant laner and especially good on Assasins, easyhoon plays better in teamfights and is a better controllmage Faker also takes more resources(sidelane farm, jungle camps, jungle ganks) and thus leaves less for the rest of the team. So it depends on what strategie they want.

Xpeke is a very reliable and save midlaner that shows up big in teamfights, Faker would have a hard time really shutting him due to how save he plays and how little resources he needs to perform in teamfights, so Sk decided to match peke in teamfights with easyhoon, makeing it a farm lane, and focus more on top/bot.

He's more like a secret weapon. When they really need him, they swap him in.

His playstyle is too reckless.

They must have Easyhoon starting.

Kind of like in American football, when one team is so far ahead that there is a very slim chance that they will lose, they can bench their quarterback and let the second string get experience while resting the starter for a more important game. It also allows Faker to keep cards up his sleeve to play later, and it is probably related to the different playstyles of Easyhoon and Faker.