Cotton is an organic fabric that collects and holds on to moisture- and when it does, grows bacteria. That's why it gets heavier and heavier the longer you sweat, and smellier the more you wear it. Technical fabrics, on the other hand, are key to staying warm and dry, because they pull moisture away from your skin and disperse it across its surface in order to more quickly move that moisture to evaporation off the material. This also helps keep you warmer in the winter, as we all know how much colder your body is when wet.
Clothing items designed for running are generally made from either synthetic materials which can be engineered to quickly wick moisture away from the skin, or merino wool, which has natural moisture wicking and anti-microbial properties.
Repeat after MIT: Cotton is Rotten!