The real reasons:
1. It looked like a cool character design so that's how and why it was done originally.
2. Not only is it a cool character design, but now, when he's talking, there's even less to move and draw for the anime.
3. After a while, it was realized that this character had never shown their face, so the opportunity to make it funny and a mystery was taken.
4. They won't ever show his face because that would ruin it; most likely this was halfway an accident that's turned into something interesting that now people are talking about.
Sources: this type of thing happens all the time in stories...too many time for me to count, so I doubt this time is different.
Lore-friendly reasons:
Those previously listed and to be listed.
