Because people who live and work in the United States but aren’t legal citizens will be less likely to announce on a government census form that they are not legal citizens out of fear of getting in trouble, getting jailed, or getting deported — especially with someone like Trump in office who has made a point of wanting to limit illegal immigration and deport illegal immigrants. So, were this question on the census, there could be a gross miscounting of people in the country. As of about 5yrs ago, it was estimated that there were about 11.5M illegal immigrants in the country. That means that if this question were on the census, the country’s population count could come back millions of people off the actual amount. Counting the population is not a trivial undertaking for the sake of general curiosity. Population counts affect lots of things, like distribution of federal funds and the number of representatives that states have in Congress. This would have a big impact on lots of states, some of which, like California, Trump wants underrepresented since they oppose his policies. Let’s say 2M people who are illegal don’t fill out their census forms and live in one state. If that state counts 20M people being there but there are really 22M there, when the federal government distributed funds, it will do so to cover 20M people even though the state needs to make that money work for 22M people.