Ember Days started out as agricultural observances tied to the seasons. Over time and as people moved to cities, the meaning got kind of muddled. As I understand it, Pope Paul VI also wanted to avoid formalism and making people fast when they didn’t really understand or appreciate why they were fasting, so he made a lot of fasting days optional/ left them up to the local bishops. One issue is that in some parts of the world, most Catholics might be poor and not eating much anyway, so making them do a fast doesn’t make sense.
People are still free to do penance and fasting themselves, but when it’s not mandated by the Church, people presumably would be choosing to do it voluntarily and thus have a better understanding of why they’re doing it than if the Church just forced everyone to fast.
