Cotton is a natural, breathable fibre that is soft, highly absorbent and generally hypoallergenic, all qualities that make it gentle on a baby's sensitive skin and good at helping regulate body temperature.
Synthetic fibres like nylon and polyester, along with delicate fabrics like chiffon, tend to trap heat, don't absorb moisture well, and can irritate an infant's skin.
That's why cotton is the standard recommended fibre for infant clothing.