Because there's no such thing as 'depressive disorder'. We expect people to be happy, and when they're not, we call them diseased. Nothing is wrong with them.
Also, Ethiopia is in the midst of a civil war isn't it?
Depression has been a problem in Ethiopia for much longer than the conflict in Tigray, despite the abundance of "community, shared culture, shared religion." Looks like it takes more than that...
I don't pretend to be an expert on Ethiopia, but I was under the impression that it's a multi-ethnic state of different tribes who don't necessarily share those things with each other.