Originally, the only reason why a census was required every 10 years by the US Constitution is to apportion members of the House of Representatives among the States, according to their (citizen) populations To the best of my knowledge, there have been no court rulings holding that non-citizens are to be included in the calculation to determine House representation. Therefore, there is a valid constitutional foundation for segregating non-citizen census responses from those of citizens. And in order to do that, the census must ask the citizenship question.
Please correct me on this if I am in error.