They don't change it because why change a winning formula, you may disagree but in my clan I've seen at least a few dozen brand new players in the last few months, that's only english speaking players and one's who have crossed my path in the clan. Blizzard don't release results but to their shareholders which they can't lie to by law, Diablo 3 is one of their biggest money earners.
I notice a lot of people complain because the game has gotten stale but have spent a 1000 hours on the game, of course it's going to get stale. D2 only had one big change during it's 12 year run, nothing else, no 3 mth patches with new items and set buffs.
I just don't get the constant complaints from a select few, is this really the only thing you have in your lives. No other game, no real friends just the desire to complain about something that won't change, you know what they say about the definition of insanity.