Well, not quite "overnight." MLK Day wasn't set until 1986, and there was considerable resistance to it from several states. if I recall correctly, many states did not honor the day as a paid holiday until long after.

It became more "celebrated" after MLK got "whitewashed" by the Radical Right Wing . (NB: I no longer call them conservatives, because they ARE RADICAL, not conservative, just radical toward the right wing instead of left.)

The Radical White Wing has removed every element of King's movement that in any way SERIOUSLY challenged the racist status quo. In that form he is used as a whip to cobber people who are actually fighting racism. They've made it about individual choice and about not using objectionable epithets about black people (at least, not in public).