There was like a dozen people "considered." Are they all by default above Raven? Just seems like sloppy logic. Wiccan is still portrayed as a tentative kid half the time. I think raven would make him pee his pants then kill him.
Wiccan's sort of "would be a good candidate for Supreme if he was 10 years more experienced." That is, he has the potential but he's not near there yet.
Wicaan is basically a boy version of Scarlet Witch....he has the capability to take this, but unless he just spazzes out and goes ape-shit insane on Raven, she probably takes it.