Because it doesn't matter who he was. What matters is who he is. Never mind the man. The name Martin Luther King has come to define the dream of equality, mutual coexistence, the dream of the death of racism. When conservatives oppose his veneration they, in the minds of many, oppose the veneration of the ideas he's come to symbolize.
Historical realities are meaningless.
This, friends, is the political reality. The Republican Congress, in an act of singularly striking stupidity, refused to condemn the Council of Concerned Citizens in an official piece of legislation. This is a group that promotes white supremacism, racial separatism, holocaust revisionism and various other theories of lunacy. To his credit, RNC Chairman Jim Nicholson did at one point condemn the group and its supporters. Yes, it's a waste of time.