This has absolutely nothing to do with art OR graphics.
It has to do with the fact that Namco thinks the main Soul Calibur game will (a) not fit with the demographic of the Wii and (b) not be able to compete with Nintendo's first party titles.
We've already seen Wii spin-off games outsell main titles a couple times, most notably with Sonic & the Secret Rings beating the much derided main title, but also with Tony Hawk Downhill Jam beating the PS3 version of Project 8.
I showed this in the last thread about SCIV... SCII sold 3.3 million copies between GC and PS2 alone. No other Soul game is in VGCs million-seller database. That means SCII outsold Edge, Calibur and Calibur III combined. What was different that time? Link. Link turned SC into a big franchise, bolstering even the other versions through the big to-do in the gaming press that his appearence created. (Let's be honest... The other special characters didn't exactly have the same appeal.) SCIII returned to less than 1 million sales, even on the massive PS2 install base.
If Namco announces that Link (or Ganondorf?) will appear in SC: Legends, and seeing that Wii doesn't have a lot in the way of traditional action/adventure type games, and seeing that there's already a precedent for it, I think the spin-off can compete with the main series, at least individually, if not combined.
"[Our former customers] are unable to find software which they WANT to play."
"The way to solve this problem lies in how to communicate what kind of games [they CAN play]."
Satoru Iwata, Nintendo President. Only slightly paraphrased.