The initial appeal for me was that I enjoyed harvest moon, except for how the old tech made the experience of playing it suck so bad, I couldn't replay it. It was annoying doing any of the basic tasks like switching tools iirc. so there was a huge opening in the market for a new harvest moon that wasn't annoying to play. And where you were allowed to be gay.

So the initial buzz came from that, imo. the people who wanted a new harvest moon game were like 'wow, finally!' and then word of mouth did its thing. these days, nostalgia for it specifically drives people back to play, along with extensive modding and occasional free updates keeping things fresh.

i think other people can explain better why the harvest moon formula itself is so appealing, but i just think it's interesting how an indie game can get so popular by just being like "what if i made this big corporation game people want a new entry from, but fixed the stuff in it that sucks?'