DkS has the best PvP of the series by far in my opinion.
Demons Souls had a really, really limited number of builds due to how stunningly overpowered a few weapons were, but it also had some good pacing and provided a solid base for Dark Souls. Mechanically, it is a sound and arguably superior game, but the balance is a God damned mess.
Dark Souls comes along, and I might get flack for this, but even pre-patch, it still had alot of variety in builds (since so many things were "OP", it provided some variety). Post-patch, it becomes a arguably more balanced and varied game that caters to people of all skill levels. I have had fun playing this game from a cheesy mid-rolling Falchion spammer, to a fast rolling quality fashion, to a knightmom Tryhard. Of course it is going to suck when you lose and its much easier to blame the game than question your own skill.
Dark Souls 2... It's a mess. The controls feel sloppy, the combat is a boring, the variety in weapon movesets has been completely annihilated and replaced with shitty powerstance (powerstance is just a moveset with varying damage output). The hitboxes are terrible, the lag compensation is terrible, the balance is not even worth discussing and then Soul memory sits atop of this shit mountain mocking everyone.
It simply lacks the longevity that DkS and DeS had. People will get bored and move on because there is only so much you can learn from the game. You will reach a point and won't really get better after that, whereas in DkS you can tell the exact moment where the fight went wrong or the exact thing that you need to improve upon. DkS2 just requires an understanding of spacing and good stamina management, and you will win almost every fight.
I think people like DkS2 PvP because the barrier to entry is much lower. Everyone basically started at the same place since the mechanics were very different and people who couldn't catch a break in DkS were able to compete on some level in DkS2, and it is just more forgiving to stick with DkS2 than crawl back to DkS and expect to get good at it.