I think it is the feel–good factor in the film that attracts people.
So they don't see beyond the ending and emotional manipulation. A sensible man is convicted of life imprisonment and he has the patience to spend 20 years in the prison but then why he couldn't stay few more years in the prison till his term ends. And if in the process of breaking the prison, he is caught then won't he would have to spend other many years in the prison. After all he is a sensible man!
So you see how the basic plot has some serious problems. Not to deny there are some other problems in the story, but it's silly to see how people can't look above the feel good factor in the film. And it is not a big deal to tell that there is not much aesthetic quality in the film.
So all in all, one can say the film is heavily overrated seeing that is ranked first in IMDb ratings. And it is nice to hear that the director of the film too agrees with this.