I don't know why people don't like it. Statistics show that it sells more manga and anime more than Naruto. Ask yourself why some people don't like Naruto and you'll understand what it's like being in the position of others that enjoy different types of anime. Naruto is all good and stuff but it lost it's edge and has never really had this hidden epic factor that One piece has. The character development in One Piece is second to non. Their is barely any filler eps and if they're filler eps then they usually better than Naruto canon eps.
I also ask myself this question "Why do so many people dislike One Piece?"
I guess it's all about what a person likes. I enjoy One Piece because it's a bit more understanding and the Protagonist actually does something to other people when they hurt His friends. He doesn't go mental, turn into a dog and start trying to kill everyone. He tells his friends to basically chill out, relax and kick back while he go knocks some heads.