D1 isn’t strictly “better”. It’s that D2 was disappointing as a sequel that isn’t working any better.
It’s like D1 was a high budget, very ambitious game, that massively under delivered, and came out with lots of compromises.
D2 is a repackage built on the same stuff, but aiming much lower. It hits its goals better, but those goals already started as less ambitious. It’s just a side step that does some things better and some other things worse. And generally feeling less significant.
My complaints about D2 are mostly technical. D2 has a worse performing, worse looking engine, with worse models, worse animations and the most downright terrible implementation of some effect like local reflections and TAA (and worse LOD and other things). D1 doesn’t have these problems and locations look generally much better with a significant higher level of polish. It looks better, performs better, plays better. And it’s absurd that a game that comes out after instead of showing purely technical progress shows this kind of regress. It doesn’t make any sense.
Then yes, D2 is better designed with loot and some mechanics. But it ends up feeling just an exp pack to D1 without the same charm and polish. It also didn’t seem as well supported post-release, probably because it also under performed even compared to their lower expectations (they even recently canceled some update that was already announced).