Why is 面巾纸 different from 纸巾?
面巾 is also a kind of napkin. 面巾 is for face cleaning, 手巾 is for hand cleaning. So 面巾纸 is usually the small pack soft paper napkin you put in your pocket, used for cleaning face. you can think it is the same thing as 纸巾. Here are many things that have different names.

This case, it is a combination of 面巾 + 纸. 纸+面巾 is also used. Though 面+纸巾 and other combinations are also logical, but less people use them.