string is a base type from C# library. Text is a type from the Unity Engine library.
Text would be used when you are trying to access a UI Text GameObject within a scene for example.
It has a property of the same name, text, this holds the actual text that you want to display, this is the string data.
In your scriptable objects you have no reference to the UI Text GameObject(s) in the scene, these are being used to store data. The data type for your story text is string.