Could be because Finnish repositories are less prone to fires and inquisitions. ;-}

You're right about "Good King Wenceslas"; per The New Oxford Book of Carols, the tune in the Piae is a spring song, "Tempus adest floridum." The site www.Spielleut.de has facsimile scans of the Piae Cantiones, including "Gaudete" and several other familiar songs, but "Tempus..." is not listed there. It might be hiding under yet another name--I haven't taken the time to check each page.