玉皇大帝为什么翻译成「jade emperor」？
这是玉帝的直译。

玉帝:Jade Emperor

玉皇大帝:Great and August Jade Emperor

玄穹高上玉皇大帝:Lofty Great and August Jade Emperor of the Mysterious Vaults ofHeavens

昊天金阙至尊玉皇大帝:Most Honored Great and August Jade Emperor of the Golden Towers ofthe Ever-Radiant Celestial Realms

昊天金阙无上至尊自然妙有弥罗至尊玉皇大帝:Most High and Most Honored Great and August Jade Emperorof All-Embracing Sublime Spontaneous Existence of the Golden Towers of theEver-Radiant Realms.

——道教术语英汉双解词典

另外，玉皇大帝确实是以玉石为尊号的神明，不过是否应该直译还是有待商榷的，毕竟轩辕黄帝英文总不能直译成车轱辘黄色帝王吧？

对了，轩辕黄帝在这本词典里被翻译为

Yellow Emperor Xuanyuan