Why does everyone say ELO...

They also flat-out called it ELO when the game released for the first few seasons. At some point Riot stopped using the term....we players did not.

Because for all intents and purposes, it's the same thing.

Because some people actually mean Elo and not MMR?

"The Elo rating system is a method for calculating the relative skill levels of players in zero-sum games such as chess. It is named after its creator Arpad Elo, a Hungarian-born American physics professor."

They use an Elo system, but then they add the step of match making. So its the same thing in this situation.

It isn't an acronym. Why do people fully capitalize it? It's "the Elo system," not "the EE ELL OH system"

elo > mmr + LP