I can understand why a X360 or PS3 owner, who also owned a Wii, would choose the HD version...
What about people whose only current-gen console is the Wii, however? Are that many Wiis co-owned with another console? I know that, statistically, NPD has shown that some 17%, at a minimum, of Wiis shared a household with another console, but that doesn't seem like the overriding element in the equation here.