The idea isn't to force yourself to play 1 champion all the time, it's to play whatever until something really clicks with you so you want to play them all the time.
For some people, that'll be more of a thematic click so while they may not necessarily be that good with the champion at first, they enjoy the champ enough that they'll keep at it. For others, they may find they're a natural at the champion and it's the confidence that they'll be able to carry that really pushes them forward, to the point they don't feel comfortable playing anything else at the MMR they end up at. Occasionally it's a combination of both that does the trick, which can push people to hard one-tricking, rather than simply maining whatever but having a few back ups.
I almost exclusively played janna this past ranked season (and much of the one prior to that as well), since I both liked the idea of protecting people and not being kill focused, and the fact I turned out to be rather effective with her. Shes since been nerfed and/or reworked a bit so I'm a little less hardcore into her thematically (where she now wants to be pretty pokey early game), but I'm still rather good at her, and I'm pretty uncomfortable playing a lot of other picks at this mmr for now. I suspect I'd do fine in reality since I've been pretty multi-faceted in normals, but it's a confidence thing.
(That said, I do play an awful lot of ARAM when not playing ranked, so that does a lot to diversify my champion experience. When I'm playing ranked rift though, it's pretty much all janna all the time.)