Free Champion Rotation Diversity

I'm a fairly new player(joined in April), but already over the last couple months there's something I've noticed about the free rotation: a lot of returning champs. Typically there will be at least two champions that were on the free rotation in the last few weeks that make it onto the roster each week. And, between them and characters I've unlocked myself, I really only have about half of them I haven't played before.

Last week, there were only three, due to me unlocking Katarina literally the day before to see what all the fuss was about. This week, there are none. I've unlocked Singed, and after a frustrating time with Hextech crafting, unlocked Jayce; the rest of the champions on the free rotation have all been on at least once(I'm looking at you, Taric) in the last several weeks.

So. I've read the short post on how the champions are picked for the free rotation, but what I really want to know is that if there are over a hundred characters of varying difficulty and play style to choose from, why do so many champions get picked for the free rotation over and over? It seems to defeat the purpose of the free rotation itself, which to me would be for newer players to have an opportunity to play a variety of champions. Given Riot's stated process of selection for the free rotation, there are some unflattering things the lack of diversity implies about Riot's champions and newer players. Anyways, whatever insight or answers someone might have would be appreciated.