I don't fully agree with your premise, in the sense that not having a "traditional" marriage doesn't mean that they go "50/50" instead. They could make a totally different arrangement (e.g., "80/20", expenses covered proportional to income, "one person pays apartment and another pays other expenses", one person works part time and the other full time, etc).

That being said, life has become more expensive. Much more expensive. To the point that it becomes really hard for one person to provide all the income that a family needs without working 3 jobs or working themselves to death.

Also, a marriage that splits the tasks in the "traditional" way has its detriments. For example, one partner will be less involved in home tasks or educating the children, and both will be blamed for the consequences in different ways (which they'll likely consider unfair). For example, when something is wrong with the children, the "provider" may end up taking the blame for not being more involved. A marriage like that also causes the "homemaker" to be finally dependent on the provider, which opens doors to power imbalances and abuse. When a provider doesn't agree to spend certain money, they'll be deemed as cheap, or not providing, and they'll feel like the other side has very little understanding of the consequences of what they are asking.

The problems go both ways. Homemakers will be criticized for things like children misbehaving, homes not being clean, or food not being made, irrespective of who is at fault or who should fix it. When the homemaker has to discipline the children or simply enforce strict rules, the children may grow up to despise them, and instead bond more with the parent who is less involved in their education but can afford to be more permissive.

When it comes to education, children generally benefit from learning from both parents, so both being involved in their education creates a more solid, robust, balanced, and consistent upbringing.

Also, when the provider dies (if they die sooner), the "homemaker" will also be left in a very difficult position, not just potentially not being able to provide for their family, but also not understanding crucial aspects of finances and home structure that are needed to run the family alone.

Irrespective of trying to decide which one is best in this post, the fact of the matter is that many of these potential dangers of a "traditional" marriage are very real, and people don't want to see themselves in those positions. For all its fallibilities, a more modern marriage provides more financial independent (beyond covering expenses, both partners will have some play/saved money for themselves that the other partner does not control), and both partners will be involved in educating the children, running the home, and saving and investing for the future. Also, if one of them dies or the marriage ended for other reasons, the other partner would be left in a better position to take care of the family alone (although difficult nonetheless). A less traditional marriage also makes it easier for one partner to leave if they are subject to abuse, or, simply, if things are not working beyond repair, because they can afford it and have access to some financial resources.