Oh I have been playing a bit of DS1 off and on recently and I'm confident in my assessment.
The biggest factors for me were level design and the feel of the world. While DS2 had more consistently good levels and nothing nearly as bad as the worst levels of DS1 (ie Izalith), they were not as good or memorable as the best of DS1.
As far as the setting, with Dark Souls you were given a very clear picture of the world and your place in it in the opening cutscene. It felt mythic. With DS2 you are basically just told "you don't know what this place is or why you're here, just get souls and stuff". There's also the interconnected nature of the world in Dark Souls. In DS2 the levels feel disjointed and unconnected, which is made worse by the fact that you can warp between bonfires from the start. I understand they were going for something closer to Demon's Souls' hub system, but in my opinion it was a step back.
There are also several design decisions I think were for the worse. Soul memory is the worst offender since it completely screws with multiplayer dynamics.
In the end though, I trust my unconscious reaction to the games as the best gauge of how I like them. With Dark Souls I was immediately hooked and went through several playthroughs just messing around with different character ideas. With Dark Souls 2 I stopped playing after a couple months. Maybe I was just burnt out, but something feels off about DS2. Aside from all the points I mentioned above, it just doesn't feel as good to play as DS1.
I could go a lot more in depth, but I think I've summed up all my main reasons. As I said, it's still a good game and I liked it, just not as much.