What's even more remarkable is that the other famous "one guy who created an alphabet" was Cyril the Philosopher, famous for designing the Cyrill— psych, no, he designed the Glagolitic script and it was shit and died out almost immediately
Cyrillic was just named after him as a consolation prize.
Why does Korea have a holiday for the alphabet?
Because unlike other writing systems that evolved over centuries, Hangul was the work of one dude who solved mass illiteracy by sitting down and creating a brand new fully-formed system from scratch that anyone can learn almost instantly.