2 reasons:
White America has been aborting and contracepting themselves out of existence.
They’re calling Afghan and Mid-East Muslims “asian”.
In this whole region, only two nations, South Korea and the Philippines, have large Christian communities. The two largest nations, India and China, are Hindu and atheist, respectively, though both have some Christian influence.
The bottom line is that the vast majority of Asian immigrants, even if not Muslim, will not share the Christian faith of our country's past. For all the industriousness and intelligence of many Asians, mass immigration from this continent will further dilute America as a nation and culture founded by European Christians.