Well I think he did it because when Lincoln was president he fought so that the slaves would be set free and would not have to work and get whipped.  And Martin Luther King had a similar purpose.  He was trying to get the rights for the black people.  So I think ha gave the speech there because he wanted to remind people that we are all equal and that everyone has the same rights.    

And ask the people how far have we actually come since Lincoln was president 150 years ago.  Have we actually fixed the way we act.  Or do we just like to think that we have changed.  And to say we need to get ourselves towards getting equal to everyone has the same rights. Also because it was really nice setting and it had a lot of space so everyone could fit.