Here's why:

* Molecular Structure: Plastic molecules are tightly packed and have little freedom to vibrate. This makes it difficult for sound waves to travel through the material.

* Density: Plastics are generally less dense than materials like metals or wood, which also contributes to their poor sound conductivity.

However, there are some exceptions:

* Certain Types of Plastics: Some plastics, like PVC (polyvinyl chloride), can be quite good at conducting sound, especially at higher frequencies.

* Thickness and Structure: The thickness and structure of the plastic can also affect its sound-conducting properties. A thin, flexible plastic sheet will be a much poorer sound insulator than a thick, rigid piece.

In general, though, plastics are considered good sound insulators, making them suitable for applications like:

* Noise reduction: Plastic barriers and panels are used to dampen sound in various settings.

* Acoustic insulation: Plastics are used in building materials and acoustic panels to improve soundproofing.

* Musical instruments: Some instruments, like drums, use plastic components to enhance specific sound qualities.

So, while the answer isn't entirely black and white, plastic is generally a better sound insulator than a conductor.