It’s extremely popular because it is a well polished Harry Potter game; something the world has been begging for for a long time. It does invoke that sense of magic that the films do so well, so if you’re just looking to get a fresh hit of the Hogwarts world it’s great. There are also a lot of really interesting and well designed characters. Most of all the world, particularly the castle, is beautifully and lovingly crafted.
The game buried beneath all that polish is a pretty basic-ass RPG. It is crammed with filler fetch/find quests. The dialogue system is just another exhaust all options non-system. The combat has some really cool ideas on paper but I personally ended up mostly mashing buttons against one of like 5 generic enemies most of the time. Also the only customization in this RPG is your appearance.
It’s not a bad game by any means, but i wouldn’t say it’s great either.