Here's why:

* Insulators resist the flow of electricity. This is because their electrons are tightly bound to their atoms, making it difficult for them to move freely.

* Conductors allow electricity to flow easily through them. This is because their electrons are loosely bound and can move freely.

Plastic is a good insulator because its electrons are tightly bound. This is why plastic is often used to cover electrical wires.

However, it's important to note that some plastics are better insulators than others. And certain conditions, like extremely high temperatures, can change the conductivity of plastics.