For me, it is because of the Voldemort backstory scenes. I love the way his history was told through the Pensieve flashbacks, the introduction of the Horcruxes, and the Astronomy Tower drama.
I know what you mean. I personally loved all the movies, but Half-Blood Prince was not my favorite of them. I don’t nitpick or criticize them at all, as I thought all eight of them were great, but I will admit I wish that they hadn’t omitted as much stuff as they did from the sixth film.