Because they're a business. They can either spend the time and money to nerf(balance) the one champ/item in question, or buff the other 10-20-50 comparable champs/items. It's just not a good use of their time to try to bring the majority in line with the minority. --- The more you study, the more you learn. The more you learn, the more you forget. So what's the point of studying?
I prefer them to nerf, If they instead buffed champions constantly to bring them in linewith the best then eventually we would start to see stuff like LVL1 single spell wave clears. There is always gonna be someone more powerful in a game like this I would rather not see everyone constantly buffed to that perosns level.
SF2 is an awful example. It's probably one of the least balanced games out there. Guile dominates everyone without even trying.
And again, just because they're nerfing champions that are played a lot, does not mean they're nerfing the right thing about them. Or "balancing". --- "So selfish them" would be their cry. And, who'd be brave to argue? Doin' what you people need is never on the menu!