Because it is mostly a regional holiday. The emancipation proclamation came several years before & the 13th amendment only a few months more. Texas was like the Japanese dude stuck on an island thinking WW2 was still in effect when it was ended several years earlier. Plus it took like hundred years more for blacks to have the same legal rights as whites, so it was at best a job half done. Kind of a pointless holiday to be honest, but no one is going to criticize another day off I suppose.
what day off?
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Kill From The Shadows. https://en.m.wikipedia.org/wiki/Idiot