The all day was set to one-fifth of the weekly price to avoid discriminating against part time workers. That principle is now largely set in stone.
It's a nice-to-have. Having had a few years of unaffordable freezes we need to respect the macro fiscal environment. European transport authorities make more of a point of encouraging people into 'subscriptions'. Once you are on a season ticket using public transport for evening and weekend journeys becomes a lifestyle baseline rather than a conscious choice.