Why is Clean Code suggesting avoiding protected variables?

Question

Clean Code suggests avoiding protected variables in the "Vertical Distance" section of the "Formatting" chapter:

Concepts that are closely related should be kept vertically close to each other. Clearly this rule doesn't work for concepts that belong in separate files. But then closely related concepts should not be separated into different files unless you have a very good reason. Indeed, this is one of the reasons that protected variables should be avoided.

What is the reasoning?