There are rural places in the US where it is an hour + drive to whatever the equivalent of the DMV office is, with no public transit. You can find similar places in cities where people may not have a car at all, with a long walk to find such an office that is only open during narrow hours.

People in or near poverty are going to be disproportionately affected by those conditions.

And just getting to the DMV does not necessarily mean you can get an ID that counts as proof of citizenship. There is no standard federal citizen ID in the US. A basic state ID or driver's license is not proof of citizenship. Even a RealID compliant ID is not a direct proof of citizenship, so depending on how strict the voting requirements are it may not be adequate.