Stockfish can't solve quick mate! Can you?

Have fun! :)

lichess.org/analysis/5bqn/p1PPprkn/4Pprb/5Pp1/6Pp/7P/3K4/8_w_-_-

1. c8=N a5 2. Nb6 a4 3. Nd5 a3 4. Nc7 a2 5. Ne8# 1-0

Stockfish got it at depth 73. Positions like this give me a headache just looking at them. It's as if someone put the board on a slant and all the pieces slid into the corner.

Stockfish is just scared with the amount of material black has :)
Btw, it's even more interesting to let black promote, so in the initial position put the black pawn on a5 or remove the c-pawn ( I like it without the c-pawn as it feels kind of excessive for a composition:
lichess.org/analysis/5bqn/p2Pprkn/4Pprb/5Pp1/6Pp/7P/3K4/8_w_-_-#0
)

Also interesting:
Why can Stockfish not find the mate with a depth of 21?
I thought it calculates all move possibilities one by one, kind of brute force.

Stockfish isn't tuned to solve studies, but to solve real game positions. Obviously full brute-force even to depth 10 is unsurmountable in most positions, so any decent engine uses lotta euristics to prune branches that are unlikely to be best. Here Stockfish just can't understand that here underpromotion to the knight is more likely to be best than to the queen (but many humans see that) and thus, not found anything good on low depth, prunes it. Probably it returns to such branches at even bigger depth, so eventually finds mate.