It's because Halle was the baddest with the biggest profile.
Only other chicks that were on that level or near it during the 90's -early 00's was Janet Jackson. After that, in no particular order, you prolly got Tyra, Toni, Nia, Jada, and Vivica Fox to make up most of the top ten. Some of the randoms that nikkas are naming just weren't big enough stars to cross over and be the universal standard for most nikkas.
Me personally....Rachael from Planet Groove and Caribbean Rhythms on BET > Halle Berry and most of the top 10 back then,
but not everyone knew who she was, but the ones who did definitely had her on that level of beauty.
at the end of the day you had to live during those times and have had those random convos about that shyt to know what was mentioned. From my experience, I've heard more nikkas use Halle as a reference than any other chick back then.