Here's why:

* Chemical Resistance: Polyethylene is resistant to many chemicals, including hydrogen peroxide. This is crucial to prevent the cap from reacting with the solution and contaminating it.

* Durability: PE is strong and durable, making it suitable for handling and storing the peroxide solution.

* Cost-effectiveness: PE is a relatively inexpensive material, making it a practical choice for mass-produced bottle caps.

While polyethylene is the most common material, some hydrogen peroxide bottles might have caps made of other materials like polypropylene (PP) or high-density polyethylene (HDPE). These materials also share similar properties and are equally suitable for this application.

It's important to note that the exact material composition of the cap might vary depending on the specific brand and size of the hydrogen peroxide bottle.