Because he wanted to be able to tell who the stupid and clever people were in his country.
题意:为什么国王想要神奇的衣服?
根据第一段倒数第4-6行:He wanted to be able to tell who the stupid and clever people were in his country.(他希望能够分辨出他的国家里哪些人是愚蠢的,哪些人是聪明的。)可知答案为:Because he wanted to be able to tell who the stupid and clever people were in his country.