exactly, why should it be harder to reach a higher rank on a more popular champ? that doesnt makes any sense.
IF more people play the champion, then they can average what a good or bad player is like. More people playing that champion can help define said champions requirements more and more to make the margins rather exact.
They likely do this instead of having set margins for every champion, its easier for them to work with and easier to be applied.
So really it wouldn't be so much that its harder to reach a rank on a more popular champion, but that its easier on a less popular champion because the margin is not as defined.
Thats my theory anyway.