This is a lot like complaining that Christmas should be celebrated on some other day, because Jesus was likely not born on that specific date. It's a lot of complaining for no good reason.
The better question is, why haven't we had a national holiday to celebrate the end of slavery before now? Why did it take a pack of racist Democrats in 2020/2021 to do something that real anti-racists, real Americans should have done already?
And for what it's worth, the Emancipation Proclamation did not end slavery in America. If anyone was taught such, the teacher was confused. It only freed slaves in newly-retaken Confederate territory. Slavery was (legally) ended by a later amendment of the constitution.