Why did Satoshi make the supply exactly 21 million. They would select 20 million or 50 million or any amount. Is there any particular reason why the amount is 21 million?
We can only know this one if Craig Wright is the real Satoshi. 
Honestly, there are so many beliefs about this one and you can read it on google, some article says its a personal reason of Satoshi, some also say that it comes from a bigger solutions and formula but none of them is accurate and sure.
There are a lot of people who already are been predicting the main reason behind why only 21M bitcoins but there is none(except the real satoshi) who knows the real reason behind it. It perhaps is a mystery which would hardly be solved even after those 21M bitcoins are all out for usage.