Because it's pretty. Because you get state of the art Unicode decoding and support. Because you can get a terminal on Linux in a first class GUI system that's normalized across all major operating systems backed by billions of investment.
What does this even mean? UTF-8 decoding is trivial. In fact it's designed to be simple: it's a prefix code that is self-synchronizing.
Rendering glyphs and composing characters is hard in comparison. But Cocoa, Qt, etc. have had great support for rendering unicode for ages now.
I have no recollection of having problems with iTerm, Terminal.app and many Linux terminals for rendering unicode the last decade or so. (Only with people setting their locales wrong.)