As each Tournament comes to pass the pool of viable/selected champions get smaller.
It's due to the poor way Riot balances and how infrequently they balance.
The main problem with Riot's pattern, is that ever since Season 2, they do almost nothing but nerf. They never buff, and they never bring champions who have been unviable for a long time in to viability.
There also seems to be some "favouritism" patterns with their balancing as well, for a more recent example; Kindred saw some time in the Spotlight as a strong jungler, but for such a small amount of time before she was nerfed twice in a row, and essentially had her early game turned to garbage. Aurelion Sol was released for two weeks, was instantly nerfed in several areas. Meanwhile, Tank Ekko top lane has been running rampant as a must ban / pick for a REALLY long time, Zed has been an extremely powerful champion and pick for ... ever, basically.
More than half of Riot's total champion pool has rarely or never seen play in Tournament games.
Due to Riot also reworking tons of Classes and Items, you've got insanely broken exploits and balance mistakes that just get abused to the point where it's either ban it, play it, or lose to it.