With cash you can't overcommit - you can only spend what you have - with card you can. If you are poor, overcommitting is very tempting as otherwise you will be going without things you need/want. But that temptation leads to debt.
True, cash is good if you lack discipline. For such a person, more flexibility leads to less because they misuse it in a way that reduces flexibility. It's a footgun for these people.
