The most direct answer to your question is, basically, the Network effect phenomenon.

Here are a few selected resources which explain and extend this perspective:

• Bitcoin - a 6-sided Market and Network Effect by Antonis Polemitis
• The 7 Network Effects of Bitcoin by Trace Mayer
• Bitcoin is Worse is Better by Gwern
• Bitcoin as Decentralized Money: Prices, Mining, and Network Security by Emiliano Pagnotta
• Bitcoin’s Gravity by Gigi
• Crypto Network Effects by Tobias Huber
• On Bitcoin Maximalism, and Currency and Platform Network Effects by Vitalik Buterin
• Bitcoin: Time Compression, Causation And Feedback Loops by Hans Hauge
• Why Bitcoin Matters by Marc Andreessen
• "It has the potential for a positive feedback loop; as users increase, the value goes up, which could attract more users to take advantage of the increasing value." by Satoshi Nakamoto