[waterphone]

They largely don't. See my other comment [1] for more specifics and sources, but the primary ID in the U.S. is a driver's license, and in poor urban areas people don't drive and so don't have an ID. They also often lack the documentation to make it easy to go get an ID even if they're able to—and in many parts of the U.S. the places to go and get an ID are only open a few days a month, which makes it nearly impossible for people with no car and who are often working multiple low-paying jobs to support their family to schedule going there. Someone who was very motivated to do so could likely manage, but it creates enough of a barrier that many don't bother voting as a result.

[1] https://news.ycombinator.com/item?id=12023285