plus OU are pretty predictable where as ppl dont know what to expect from the UU meta game (usually)
EDIT: Let me elaborate a bit on OU and UU if i may. Whenever a person is trying to create a strong team, they must create a roll for each pokemon in there team, be it support, or offense, or just tanking hits. As such there are many million of combinations to choose from for a team, makeing whatever you want do whatever role you need. Now of course certain pokemon are better at certain rolls than other pokemon. And as such, OU is generally set as being the "best" for the most common rolls. This though doesnt make them better than another pokemon, it just makes them best for a certain niche (although some pokemon can fill many niches).
I guess what im trying to say overall though is... Any pokemon really can be a potent team member (yes even magikarp), it just depends on how much you want the rest of your team to help it out and what you want it to do. OU is set as OU because they are the easiest to use at filling out their roll.
If that made any sense at all ._.