You don't even need to go there--many different populations are unlikely to return their forms. And the Post Office was having issues due to DeJoy and dorks.

The Census normally tries very hard to find people. They do this by sending out legions of people in person to people who may not have returned their Census forms.

The "in person" part of the census was heavily compromised this year due to Covid even before you start to add in the Republican fuckery.