Because of the bad optics of this, the US census has traditionally been extremely independent from law enforcement. The government has recently been attacking this barrier e.g. with the citizenship question
Two points on history and tradition. The census was originally carried out by the US Marshals. That should indicate the importance and weight of the project. The citizenship question was part of the census until 2000 (1950 short form, 2000 long form).
If you are using the census to distribute voting power to states, and the distribution is based on count of citizens, not people, then it is a fair question to ask.