Satoshi didn't explain it very well other than that he put allot of care into it. He mentions its a 64bit value, but nothing about 21x10^14 being about half the maximum representation of a positive integer by a 64bit float.

I suspect 21 was chosen based on a convergence of low risk of improper computation due to overflows when adding or subtracting two sums, and a coincidence of 7x3 which are two "perfect" numbers.

7 represents God, and 3 represents the physical universe. One could say that 21 is God's perfection manifested in the universe, or said another way; Bitcoin is condensed energy in the information realm akin to a Szilard engine

It's interesting to see how Mike's Bitcoin journey started. His later long and in-depth Bitcoin articles have positively influenced mine. I've always enjoyed reading his blog posts, whether or not bitcoin related...

I notice his address still holds the 82 bitcoins. I thought he had gotten rid of all his bitcoins when he left the project, or probably, he just lost the key.

I find this answer better:

Should have been 42

будь внимательнее! 21 000 000 это 33 халвинга .Возрост Иисуса