I never said it was. I just said it was darker. :P
That's fine. I was just saying. :T
Personally I don't find P3 to really be that "dark" either. It has some dark aspects to the plot, but those get washed away by the overall hopeful tone of the game and the ridiculousness caused by trying too hard to be dark (for example the amount of blood on the ground during the extra hour makes me laugh more than it makes me feel uncomfortable).
What makes a game dark is the thematic and how that it's presented with the scenario and characters to the player more than anything. But is also important to notice that "dark" in itself is as subjective as what horror is (or means) to each person.
That said, the fact that I ended up seeing P4 as a better experience with the anime adaptation than the game(s) tells a lot about the way I view P4 overall.