Riot Champion Balance in a Nutshell

An excerpt from an explanation on Nerfplz for why Twitch is really strong right now:

There's nothing particular about this patch that makes him stronger, but Youmuu's Ghostblade buffs from long ago combined with many marksman nerfs have led to this inevitable cycling of marksman.

Riot has never fully grasped the concept of a power budget and as a result, Marksmen (and this goes for all other Classes as well) have never had the same power budget as each other. The amount of power they have in the budget has always been in a constant stat of flux. The reason? Riot very rarely tweak champions.

For the vast majority of the time, they either Buff or Nerf champions. But it's not like Riot has the resources to keep all champions with a proportionate amount of power compared to one another. That means that there's always 3-5 champions that are really strong and all of the others you can't justifiably pick 9 times out of 10 because the Effort vs. Reward ratio isn't favourable enough.

Riot's has always been fairly single-minded in their balance philosophy. They've never been the type of people to try to shift some of a Champions kit from one ability to another or one type of power (i.e. Mobility, Damage, CC, Utility etc) to another.