Why did Satoshi choose 21 million?

Why did the anonymous creator of Bitcoin, Satoshi Nakamoto, decide to cap the total supply of Bitcoin at 21 million? Was there a specific economic rationale behind this decision? Was it an arbitrary number chosen to make the digital currency seem scarce? Or was there a deeper reason, related to the technical underpinnings of the blockchain or cryptography, that influenced this choice? How does this limitation affect the potential for Bitcoin to serve as a medium of exchange or a store of value in the long run?