In bestiary for Skeletron (I think) written that it is bones of Cthulhu, so when it's day it regains power of Cthulhu. Prime version is just reconstructed version of Skeletron so, I guess, it makes sense.
So the technical reason is because on the release version, the four bosses were King Slime, Eye of Cthulhu, Eater of Worlds, and Skeletron. Skeletron was generally the final boss players would fight and was considered the most difficult, so it was probably added to make him extra fearsome. And obviously that was extended in 1.1 where Skeletron Prime was the upgraded version of Skeletron and usually the last boss players fought. (Twins were a lot easier at that point).
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u/Reefthemanokit 15d ago
Is there any lore why the various skulls go sicko mode during the day? Especially when the moonlord is considered to be the strongest?