I think the general "consensus" I've noticed over the years of why people like S1 or S2 more can be boiled down to this:
Those who prefer Shenmue 1 like the feeling of having a smaller, intimate setting where they're familiar with every inch of the map and are more connected to the world. They like the feeling of existing within a small town and enjoy how much interactivity they have with the environment. It's much easier to notice details such as the same NPCs in different parts of the environment during different times of the day and in general there are a few more details that make it feel like a lived-in world. I think there's also the factor that it's the first game in the series and what it did was completely new, and that feeling of "newness" could never be replicated in a sequel.
Those who prefer Shenmue 2 tend to, of course, like all the same elements as Shenmue 1, but the feeling of a familiarity is lost since you travel to many new and unfamiliar locations. However, that familiarity is traded for a sense of unease and curiosity that's a reflection of where you are in the story. You can spend a lot of time exploring since the locations are much larger and there tends to be more payoff in finding a few shops or gambling locations off the beaten path. There's a much larger and ambitious story which moves at a faster pace along with an increased amount of combat encounters, branching QTE's, set pieces, and boss fights.
And yes I know that I don't speak for everyone but those are the common thoughts I've noticed. Shenmue 2 is my favorite game of all time and while I really loved S1, I just think S2 is all-around better. I definitely appreciate the small town feel and attention to detail of S1, though.