How it wound up on Christmas playlists is beyond me.
The real answer though is the number of artists who've covered the song on Christmas albums over the years:
1964: Jack Jones on his album The Jack Jones Christmas Album[2]
1965: Eddie Fisher on his album Mary Christmas[3]
1965: The Supremes on their album Merry Christmas
1965: Andy Williams on his album Merry Christmas (He also sang it in a duet with his wife, Claudine Longet in 1966 on The Andy Williams Show)[4]
1966: Kenny Burrell on his album Have Yourself a Soulful Little Christmas
1967: Barbra Streisand on her album A Christmas Album
1968: Herb Alpert and the Tijuana Brass on their album Christmas Album (reached #45 on the Billboard 100 the following year)
1968: Tony Bennett on his album Snowfall: The Tony Bennett Christmas Album
1969: Johnny Mathis on his album Give Me Your Love for Christmas
1969: Peter Nero on the album Christmas with Colonel Sanders[5]
1970: Rick Wilkins and the Mutual Understanding on their album Christmas with Rick Wilkins and the Mutual Understanding[6]
1981: Kenny Rogers on his album Christmas
1984: The Carpenters performed an instrumental version on their album An Old-Fashioned Christmas
1993: Lorrie Morgan on her album Merry Christmas from London (Christmas airplay in 1994 and again in 1999, bringing it to #64 and #69, respectively, on the Hot Country Songs chart)[7]
1994: The Whispers on their album Christmas Moments Sampler[8]
1995: Luther Vandross on his album This Is Christmas
1995: Russ Freeman on his album Holiday
1997: SWV on their album A Special Christmas
1997: Petula Clark on her album Merry Christmas...Joyeux Noël[9]
2000: Vanessa Williams, Plácido Domingo, and Tony Bennett on their Live concert Christmas special, Our Favorite Things: Christmas in Vienna[10]
2001: Perry Como on the album A Perry Como Christmas[11]
2002: Barry Manilow on his album A Christmas Gift of Love
2002: Anita Baker on her album Christmas Fantasy
2004: Dionne Warwick on her album My Favorite Time of Year[12]
2005: The Brian Setzer Orchestra on their album Dig That Crazy Christmas
2005: Kenny G on his album The Greatest Holiday Classics
2005: Rod Stewart on the album Sounds of the Season: The NBC Holiday Collection[13]
2007: Yolanda Adams on her album What a Wonderful Time
2007: Connie Talbot on her album Over the Rainbow
2008: Tony Bennett on his album A Swingin' Christmas (Featuring The Count Basie Big Band)
2008: The Brian Setzer Orchestra on the album The Best Of Collection – Christmas Rocks!
2009: Family Force 5 on their album Family Force 5 Christmas Pageant
2011: Deana Martin on her album White Christmas[14]
2011: Carole King on her album A Holiday Carole
2011: Chicago on their album Chicago XXXIII: O Christmas Three
2011: The cast of Glee on Extraordinary Merry Christmas
2013: Kelly Clarkson on her album Wrapped in Red
2013: Jim Brickman on his album The Magic of Christmas[15]
2013: Mary J. Blige on her album A Mary Christmas
2016: Leslie Odom Jr. on his album Simply Christmas