Remember it's important for Riot to stay objective and eliminate as many unreliable data sources as possible when choosing to update champions. Often times when people claim a champion is overpowered it is because they are less skilled than the enemy or don't know how to properly fight against said champ. Likewise, when others claim a champion is weak or under powered it is usually because they are not as skilled as their enemy or don't know how to play that champ correctly.
Certainly this is not always the case, but in order to make informed good decisions it's best to look at data from sources where skill and game knowledge are much less of an issue. Aka High Elo/pro games. Usually, if a champion is off-balance that will show itself pretty quickly in high-elo games.
The downside to all this, is that while champions can be perfectly tuned for high-end play, they could be horribly oppressive in lower tier games. Changes made for high-end play can often ripple through the lower tiers and cause problems. Riot doesn't ignore these issues, they just want to eliminate whether the issue is skill/knowledge, or if infact the champ/ability really needs adjustments.
This is particularly an issue when champs are first released or re-worked. Game knowledge and skill of how to fight as/with/against said champ basically gets a reset throughout the community, so Riot typically waits quite a while before making significant changes to that champ; Effectively allowing the community to learn and catch up before they can really gauge whether or not they are balanced.