First of all, one of the main reasons why poeple chose films is that their faster and easier to "consume". In oder words, some people would rather spend 2 hours watching a film than 4 hours reading the book,
Secondly, the film industry has become one of the main conveyance of entretainment, as most people consider them to be way more engagin (at least in most of the adaptations, as there are also some companies that are eager to hire slothful directors to ruin someone else's work).
Regarding "To Kill A Mockingbird", as someone who has read the volume and then watched the film, they are both astonishing works. I was, indeed, startled when I fist saw each of them as I was completely unaware of how phenomenal and marvelous they were. Therefore, it is beyond doubt that anyone who's seen any of those adaptations won't be needing to see the other (and most people won't as they already know the plot), as the author's technique, the character's personas and the actors unbelievable performances are truly staggering.
In my opinion, anyways, some of my reasons why books are a better and more pleasing entretainment (although I still relish many movies) are that they offer a deeper backround and story development (they are not limited to fitting the length of 2 hours); and I have a greater time reading them than watching the films.