One crucial factor in Stockfish becoming a leading chess engine is its test framework.
Summary: https://en.wikipedia.org/wiki/Stockfish_(chess)#Fishtest
Initial announcement: https://www.talkchess.com/forum3/viewtopic.php?start=0&t=47885&topic_view=flat&sid=b5662f579ad4c7bea91668bb9d9723a4
Lots of engines have clever programmers, but a strong test technique gave it an extra edge!
