There have been countless examples of why it's so beloved.
I only remember people praising its approach to NG+ (a lot of random red phantoms everywhere, the only good thing is the Freja encounter, which actually would made the base NG way better and there's no sense locking it behind NG+, clearly shows their lack of time when redeveloping the game), PvP (I can understand people liking a lot of variations of equipment they can use, even though it's mostly visual stuff and I find DS2's visual style very weak) and DLCs (DLC content is actually very good, I agree here, they made the game way better).
I understand that there was a large backlash when it was released and the game was hated a lot for superficial reasons, but I feel that the recent wave of fanboyism towards DS2 is the same, but with opposite sign. I mean, people are free to select their favorite game, but DS2 fans are really obnoxious online, trying to cling to Elden Ring as if that game owes DS2 its success. DS2 fans have a trauma and it's not healthy.