You people have never seen Maractus, have you?
But in all seriousness, it just takes a considerable amount of working to get certain Pokemon to work OU. I wouldn't write them out of OU usage just because of it either. Take my favorite Pokemon Dewgong for example, he's universally known as shit and is often seen as an inferior Walrein. However, when I've used it in OU and even Ubers (Fuck Manaphy, Dewgong OP<3), I've had moderate success. It all boils down to how you attack it. If you know that the Pokemon you're using is on the weak-side, it's all about planning. When, who, how do I plan for x, etc. All fall in to place if you're going to bring something from LU or lower into OU. With planning, anything is possible. There's a reason why I still use Maractus as my goto gal. Maractus > Kingdra, ask Owen.
Then there stands the issue of people who walk around acting like they're Jesus. H. Christ for using an "original team". "I use all of my favorite Pokemon. OU is so unoriginal and takes away from the fun of the game." Kudos them for deviating from the standard OU team, but being original just so you can say that you did something original or doing just for the sake of having others see you as original is, well..... unoriginal and stupid. At the end of the day, this game is competitive, and people like to win. In order to win, most will flock to the strongest Pokemon. It's been that way since Gen I when you couldn't go to a tournament without seeing the standard -Alakazam - Slowbro- Mewtwo- Chansey- Starmie -Gengar- teams. That's why we have tiers. That's why we have a ladder. However, If the FEAR strategy that tore up Wifi battling last gen has taught us anything, it's not to underestimate any Pokemon, even if they aren't viable in their own tier.