Q: Why did Satoshi choose an upper limit of 21,000,000 BTC?
A: The limiting factor is the max value of a 64 bit integer (18,446,744,073,709,551,615) and 21,000,000 BTC is represented as 2,100,000,000,000,000 satoshis.
https://t.co/lUo4fRn0yS pic.twitter.com/KbxSyesiNK
However, I don't think there is any particular reason.
More important than "Why is 21 the limit" is that there is a limit after all.