There are a lot of strong champs who never see time in the LCS (excluding once in a blue moon) for whatever reason. Some it is because they have a weak team fight or are just to weak In the early game without a crazy late game. I think one of the main reasons a champ is not pick is because they have no dash, jump, etc. I would love to see more champs other than the generic Lucian, Thresh, Lee Sin, TF, and Ryze.
To answer your question: when they are** comparatively** stronger than other champions.
Champion diversity is something that people has wanted for a very long time. The current Preseason changes increases complexity in the jungle, and it will be interesting to see how it affects jungler diversity.
Adding healthy complexity to the game through the map and champions is something that Riot is experimenting with, and it should allow for more champions to be competitively viable. There will always be comparatively stronger champions however, but when you have enough complexity (aka factors that can affect the outcome), optimization becomes more skilled based and harder to achieve.
Ryze is popular champion for simple reason that he is comparatively easier to optimize while doing a lot of things well. The problem with top lane in particular I feel is the limited factors when it comes to optimization, so unless that is addressed, there will always be another Reneketon, Ryze or Tree.
