That's a great question.
One of the challenges when writing any kind of Internet regulation law is that technology moves very quickly. For example, right now the Europeans are suggesting things like banning addictive features. In technology, it can be really hard to define what an addictive feature is. If I say, “This is the thing you're not allowed to do”, what usually happens is that either the definition is specific enough to be easy to understand, in which case the tech companies immediately just do a slight twist and say, “Well, it's not in anymore”, or you have a situation where you write them at such a high level that you have to ask what it means to have an addictive feature.
Duty of care is a nice, flexible in-between where you say, “Hey, you need to be demonstrating proactively that you're looking out for the needs of children and designing safety by design.”