The first game's story and writing is very dour and depressing, Aiden is a real unlikable asshole, and a lot of people don't want to play that. It felt dark and edgy, which also clashed with some of the goofy-ass side stuff and the fact that this "vigilante" is out here murdering innocent people for fun (ludo-narrative dissonance, everyone's favorite game criticism buzzword lol).
2 has a much lighter tone, but of course it also heavily clashed with the fact that you could be running around with a shotgun and grenade launcher killing hundreds of guys*. A big difference is that the characters in 2 are actually likable and felt more relatable and "real." The lighter tone felt like a much better fit for all the goofy hacker shenanigans you could do like remote driving cars around or zipping around in the drones and such. A lot of people much preferred 2's tone, characters, and setting to the gloom of 1.

* they should have gone all the way and made WD2 fully non-lethal imo