Here's why:

* Structure: Cardboard is made from compressed fibers of cellulose, a material that doesn't readily allow the flow of electrons.

* Resistance: It has high electrical resistance, meaning it opposes the flow of electric current.

* Practical applications: Cardboard is commonly used in electrical applications where insulation is needed, like in electrical boxes and packaging.

However, there are some caveats:

* Moisture: If cardboard gets wet, it can become a better conductor as water can carry electric current.

* Conductive coatings: Some cardboard products might be coated with conductive materials, which would make them more conductive.

Overall, for most practical purposes, cardboard is considered an insulator.