I don't get it. I get it has lots of good Pokemon but still.
ETN isn't wrong, but there's some more that deserves mention:
It’s because it contains the most popular Pokémon that people like to use, attracting many new and experienced users. It offers freeedom, but not enough that you’d find Groudons and Kyogres everywhere. Also, OU isn’t the most popular tier, randoms is.