Answer:
I think that people found difficult to accept zero as a number because people think that zero doesn't have any value. In a way yes Zero doesn't have value but zero is also a very important number because sometimes we need to solve some questions and we need to add value but if we need to add that particular value we should even subtract that value which makes it zero. So I think the reason is people don't think zero has any value. But in my point if view zero has many values.