Because black men taking white women is largely a porn meme, white MEN are more obsessed with than white women. (If only white women in the US had voted, Trump would still be President. This is also true if only young white women had voted.) But white men fucking Asian women in and outside of Asia is anything but a meme.
And white men are super autistic about it. They've created this meme now, you occasionally see it pop up even on reddit and tumblr, hardly places for the hard right to gather, about how immigrants are selfish for coming to the West, while they themselves would feel terrible if they'd live in a foreign country and came to realize that they aren't welcome there.
I was in a debate about this meme on r/DebateTheAltRight recently and the alt-right guys were all patting each other on the back about how white people respect other people and countries and would immediately leave if they weren't welcome, while those of us pointing out the history of colonialism and linking to stories about how white guys behave in Asia (they often come there with this savior complex, about how they have to bring sexual liberty to the oppressed women of Asia who don't know yet what orgasms and oral sex are) were all downvoted, so they didn't have to see us anymore.
It's like being in a lose-lose position: they are either hardcore racists who follow a fertility cult where everyone who cannot make Aryan babies is considered subhuman, or they want to colonize the Asian means of reproduction.