Not American, but from my understanding it is because US citizens don't have mandatory ID documents. If getting documents necessary to vote is voluntary but requires time and money, going to an office in the next bigger town, bringing documents not everyone might have, ..., disadvantaged people have a harder time to obtain them and are less likely to have them.
If you get ID automatically or are required to hold it in other parts of life, it's less of a hurdle.
If you get ID automatically or are required to hold it in other parts of life, it's less of a hurdle.