Because of her looks. She played in that time, maybe around early 2000s where society was more patriarchal. When the society is more patriarchal, a woman's talent is less valued and her worthless qualities is more valued. Since Anna Kournikova is a Russian woman, she has less physical strength to play tennis than other women. So she took the advantage of her looks to rise.
If she played in present time, I don't think any person would ever value her because today's people are more intelligent and sensible than before. I know she has looks, but in reality she is worthless like Maria Sharapova. The more beautiful a woman is, the lower is her physical strength. Anna is a skinny woman so she couldn't play tennis so well like Serena. But in reality, ugly women like Serena win the game. Skinny girls like Anna should play gymnastics or skating where no "Strength" is required.
RUSSIAN LADIES ARE KNOWN MORE FOR THEIR LOOKS THAN THEIR TALENT!!