Inherited from naval wargaming, where it came about because first rate ships of the line had better armor than second rate etc. so armor class scaled inversely. That meant THAC0 was the best way to figure out what you needed to roll to get a hit.
It’s also not functionally that complicated (your THAC0 minus target AC), just weird and confusing if you try to understand why it works that way.
I would not wish Advanced D&D THAC0 mechanics on my worst enemy
all these years later and i still cant fathom why they went with an inverted dc scale
Inherited from naval wargaming, where it came about because first rate ships of the line had better armor than second rate etc. so armor class scaled inversely. That meant THAC0 was the best way to figure out what you needed to roll to get a hit.
It’s also not functionally that complicated (your THAC0 minus target AC), just weird and confusing if you try to understand why it works that way.