Imagine what Black's last move had to be to get mated like this.
Suicidal yes, but can you blame him? He was lonely, with no prospect of resurrecting his queen, his faithful guard and clergy gone, his castles removed from his power, and no pawns to make him sandwiches.
What is left to live for.
This is a sad sad puzzle...
Well, imagine that white sacrificed a bishop the following way. It would ofcourse still be better for black to move away from the corner, but at least it is more understandable that black went into the corner:
A more logical mate in one is shown in the next diagram, but it remains too easy.