Based on the iconic nature of all 3's careers, Xtina's voice alone trumps the other 3's talents combined.
Based on stats the same applies -
This is the list of females of the year according to the Billboard year end charts since 1980 (year by year):
1980: Donna Summer
1981: Pat Benatar
1982: Olivia Newton-John
1983: Laura Branigan
1984: Cyndi Lauper
1985: Madonna
1986: Whitney Houston
1987: Whitney Houston
1988: Debbie Gibson
1989: Paula Abdul
1990: Janet Jackson
1991: Mariah Carey
1992: Mariah Carey
1993: Janet Jackson
1994: Mariah Carey
1995: Sheryl Crow
1996: Alanis Morissette
1997: LeAnn Rimes
1998: Shania Twain
1999: Britney Spears
2000: Christina Aguilera
2001: Jennifer Lopez
2002: Ashanti
2003: Christina Aguilera
2004: Alicia Keys
2005: Kelly Clarkson
2006: Rihanna
2007: Fergie
2008: Rihanna
2009: Taylor Swift
2010: Lady Gaga
2011: Adele
2012: Adele
2013: Taylor Swift
Look at all those massive icons who could own a year more than once. If the supposedly overshadowed Christina could do it twice, why couldn't Britney do it twice? Why couldn't Beyoncé do it at all?