Here's why:

* Cotton is a natural fiber: It's made of cellulose, which is a long chain of sugar molecules. These molecules don't readily allow the flow of electrons (which is how electricity flows).

* Cotton is a good insulator: Insulators resist the flow of electricity. Cotton fibers are porous and have a lot of air trapped within them, which further inhibits the flow of electrons.

Important Note: While a cotton shirt is a poor conductor, it's not a perfect insulator. If a very high voltage were applied, it could still conduct electricity. However, for everyday situations, it's safe to consider a cotton shirt as an insulator.