It isn't inherently better in any way. Essentially, it is just an inversed form of BAB.
Rather than counting upwards from 0 and using a positive AC progression, THAC0 counts downwards from 20 and uses negative AC progression.
Statistically they mean the same thing, they are just separated by their individual quirks. For example (as best I recall), 2E considered -10 AC to be the absolute limit of Armor Class. No character could have an AC value better than -10. 3E allows you to reach any level of AC. You could have a character with AC 60 in 3E. In 2E, your absolute best is -10 (by the RAW).
So, neither is "better". They are just 2 different methods of getting the same statistical end result.
What tends to trip people up about THAC0 is that not all modifiers move in the same direction. A positive number to hit from attributes and magic is good. The higher the better. But Armor Class is the opposite and negative numbers are good. The lower the better. That tends to confuse people.