probably so thye audience wouldn't focus on the preference of East Asian women for white men:

One of these changes, suggested by Asian American actress Constance Wu, was to remove dialogue originally in the book from the screenplay. Specifically, when her character, Rachel Chu, “boasts about never dating Asian men,” THR reported.

Source: https://www.washingtonpost.com/news/morning-mix/wp/2018/08/03/asian-ew-gross-how-the-crazy-rich-asians-movie-could-help-change-stereotypes-about-asian-men/

BTW this film would never see the light of day if the lead was a white woman with an East Asian man. Ever.