I think that it's because of the various characters with various interesting personalities and problems that people can relate to..
And Kishimoto really succesful in creating a whole intriguing new world with many new terms and rules that not existed/related anywhere on any place in our world...it's like the Harry Potter thingie, people just get sucked into the world created by the author..
But for me, beside all the reason above, there's still one most important reason....KAKASHI
