An average movie

Because the march on Selma was removed from that speech. It's only a two hour film, they have to focus it on the specific story they're telling. They maybe could have started the movie with it, but it would have overshadowed the rest of the film. Imagine sitting down to watch a movie about Martin Luther King Jr. and that film starts off with his most famous speech. Isn't the rest a bit anticlimactic?

EDIT: just read on Wikipedia that MLK's estate had licensed his speeches to Spielberg/Dreamworks for a possible project. So, copyright. It's a shame that a filmmaker would block another filmmaker from using those words.