Because the same reasoning behind that statements implies that certain races are innately inferior to others. You chose to write "Asians" and "whites" here - why not make the same statement with "whites" and "blacks?"

Saying "Asians" are intellectually superior to "whites" is a thinly veiled way to say "and whites are superior to all other non-Asian/white races."

And the claim that "Asians" are intellectually superior to "whites" isn't even correct "because of race." I'm not aware of any real study that attributed racial identity to measure intelligence. Cultural differences? Socioeconomic differences? Country of origin? Sure. Race? Used as a proxy for the former.

Racial identity is a good proxy for ethnicity which studies do show affects IQ.