Because NOBODY talks about the ACS for some gods damned reason.
It's because
who cares. The question is simple: turnout. The Census isn't content with getting 3.5 million people, it tries to register
literally everyone. Thus, anything that lowers the response rate is bad.
When a survey is going for a subset of the population, it can afford to be selective; when it's primary purpose is to count people, it is counter-productive. Hell, I remember reading guidelines for census workers which read "knock on a door; if someone yells at you to leave, mark one person as living there and move on." When you have to have everyone, the lowest common denominator of what the entire population will put up with is already pretty darn miniscule (consider that in places where voting is mandatory, there
still are some people who don't vote.) Getting
everyone to do something is nigh on impossible, and doing that is precisely the job of the census. The simple fact is that there'll be some immigrants who see that question and decide not to answer it at all. And that will lead to undercounting. Which is a Bad Thing
TMThe fact that the Census is increasingly underfunded and being forced to skip 2/3 of their usual practice runs doesn't help matters... And now you see why I worry.