Not JetBrains, but... Because the type change due to a smart cast is a very non-obvious thing that happens in your code, and there's no real way that you yourself could explicitly make it more visible. So the syntax highlighting does it for you.

I can't say from JetBrains perspective, but my opinion is because you have declared that variable first as String? for example, now one would have expected that it keeps it type but instead type is changed to String using smart cast, in this case its simple, but sometimes you might have bigger hierarchy of sealed classes for example, where different classes have different methods and this should help you remembering that type isn't same as declared, this is maybe their way to say it used to be X type, but now it is Y inside this context.