Spoilers ahead!
It's due to the fact that DSII is a mess, the lore in it is ass compared to Dark Souls. The bosses are mostly boring and uncreative. Very few are memorable. As a stand alone, it is a great game IMO. As a soul's sequel, its abysmal. Honestly, so many of the areas in DSII just flat out sucked. I like a few bosses. I grinded out the Giant Lord over 90 times for a stupid amount of leveling. I think i'm like level 500+..
Do I hate it, not at all, but I definitely didn't get the cool feeling I got from the first two games. Demon's Souls is still my favorite of the series. Nothing was cooler then Shrine of Storms, and the bosses in Dark Souls were amazing. Ornstein and Smough for example, and the Four King's fight. IMO, nothing in DSII is cooler then them. In fact, when I beat DSII, I thought 'is that it?'. I was sort of amazed at that last boss. She will NEVER be Gwynn, or the bad ass trek in Kiln. She was a joke IMO, and that doesn't change much through repeat playthroughs. I need to get the DLC's, but again, Miyazaki was missing. So this game lacked the heart of his life's interest that makes the others amazing games. I still cringe at how awesome Shrine of Storms is.