This question's pretty simple really. A concept of the number 0 is a major landmark that's used when discussing advanced civilizations in pre-modern history. It was something that civilizations came to use after thousands of years and was a "major advancement".
3 - 1 = 2,
2 - 1 = 1, and
1 - 1 = ... well, 0. To be clear, I understand that before this, there still would've been concepts such as
null or, probably much more universally,
there isn't/aren't any. Furthermore a concept of negative numbers seems less natural and intuitive, so I don't put that in the same category. However,
0 is something that is quite visible to the naked eye (as when you run completely out of food, which is something Ancients were in routine fear of). Really, not having
0 seems like quite the elephant in the room, especially among civilizations with full-blown writing systems and reasonably sophisticated architecture, and the reason that this was so difficult to attain to scientifically seems like an elephant the room as well.
Why was the concept of zero so difficult to come by?