Why do we have a Martin Luther King day?

I am greatful for all that MLK did for colored people in america. Yet arguably two of the most influential men in american history(Lincoln and Washington) get their birthdays squished together into "Presidents day". Why does MLK deserve his own holiday? Should Henry Ford get a holiday for making the automobile widely available? Or perhaps Jonas Salk for saving america from Polio? I don't see why MLK is that important to get his own holiday if others are not.