Because even her closest friends knew of her as Black for most of her life. If you guys were old enough you'd know the 80's and 90's were a really crazy time in terms of race relations. Black people were always on the defense. If she was really running from Black identity, she wouldn't have attended an HBCU or joined a Black sorority at a time when it wasn't even cool to be Black. It just doesn't make any sense, what you're saying, and then there's this:
Identity is complicated, and she shouldn’t have to choose just one.
www.vox.com
But her multiracial identity shouldn’t have been new information. Harris has had a long career in public life: She was a San Francisco district attorney, the California attorney general, a US senator, and ran for the Democratic nomination for president last year. And yet for years, she has primarily been identified as a Black woman in the public eye, with her South Asian identity rarely mentioned in media coverage until fairly recently.
Some of this could be a personal choice — a longtime friend of Harris’s told the Washington Post in 2019 that he had only learned she was also of South Asian descent after knowing her for 15 years.