$G$ is an abelian group, so let $a,b\in G$ be given, and fix $n$. Then $$ (ab)^n=(ab)(ab)(ab)\cdots(ab).$$ Because $G$ is abelian $$ (ab)(ab)(ab)\cdots(ab)=(aa\cdots a)(bb\cdots b)=a^nb^n.$$ This follows naturally from the definition of commutativity.