demand.
it's a self-fulfilling prophecy ... for now. People know Bitcoin goes up in value, driving up the demand and price. This further makes it more desirable to have.
This is the 21st century version of the tulip craze. In the end, tulips were found not to be worth the exorbitant prices, so the price crashed.
The same will happen with Bitcoin. Unless there is a real value to it. If it can prove to be a stable store of value that increases in value over time (since fiat is losing value over time), and provides other benefits such as anonymity, quick/cheap transfers, then it could become widely adopted and justify its value. It would literally be digital gold.
However, in the end it is a ponzi scheme. People only want it because it makes them money. No one really uses it for transactions. That's why this is the perfect time to invest in Bitcoin. In the past, it was very risky and unproven. It wouldn't have been shocking to see Bitcoin fail and go to zero. It has withstood those shocks, and now has been validated and even embraced by politicans and investors. However, Bitcoin's returns diminish with each cycle. Perhaps this is because the risks are becoming less, and the law of greater rewards for greater risks applies. In additon, there is a good chance Bitcoin will fail long term, as the next generation of cryptos could eliminate the need for Bitcoin.
Take a look at this graph and decide for yourself what trajectory Bitcoin will now face, given it's past history. The current cycle is in yellow, and the previous three cycles in the other colors.
Comparing the performance of Bitcoin across different halving cycles
www.bitcoincyclescomparison.com