I can't speak for others, but at least for me, part of the reason I like Kid is because he is the perpetual underdog. Unlike Luffy and Zoro, Kid isn't chosen by god. He fails all the time and ultimately, won't achieve his main goal of becoming pirate king.
I just find that so much more real than Zoro, where the entire point of his character is just to do big feats. That's it, more or less.