CBSE Class 9 Answered

Normally sweaters are made of materials like wool.It keeps you warm as such materials are natural insulators. For example Wool traps warm body heat between the fibers, it is naturally water repellent so it can withstand moderate rain and snow, and wool is wind resistant.This is the principle on which sweaters work.

Continious washing results in degradation in quality of  cloth(or arrangement of fibre) and hence leads to a natural reduction in ability of sweaters to keep you warm.