I talked to Supercell’s Stephan Demirdjian, Hay Day game lead for the past nine years, and Camilla Avellar, game designer on Hay Day for seven years, about this success. They talked about how the game evolved over time and how it continues to get people to come back...
Since I also play Clash of Clans concurrently with Hay Day, I check CoC's in game credits and saw Mr. Demirdjan's name which suggests to me either he worked on CoC before joining HD or he is working currently on Team CoC.