The reason is because it will cause loopholes if a user can edit their post at any time.
For example, if a user is selling something for 70 dollars and changes it to 80, but a user agreed to buy it for 70 before it was changed, we cannot trace the history of it being changed.
Thus, there is a timeframe for editing a post.
What post are you trying to change? We'll have a look into it.