Going to the bank costs something, if not the opportunity cost lost. When you are poor, every opportunity or cost matters much more. Most banks require some form of ID or address verification, which again could require travel somewhere. Finally, even if you assume the most seamless, easy to sign up online-only bank, you are assuming a stable, reliable way to access the internet, which again, can be issue if you are poor.
You also have to spend time putting in our card, then your pin, then signing something, all the while waiting on their slow connection on each step. IMO, it's faster to use cash in most cases.
