Because the supply of bitcoin is limited, bitcoin is only 21 million forever, anti-inflation, because it cannot be printed carelessly like fiat money. Accessible to anyone, anywhere, transparent and cannot be manipulated. That is why we can see now, when the financial system begins to shake due to the uncertain global economy due to war and so on, Bitcoin remains strong and even continues to increase.
The comparison of strength between fiat currency and Bitcoin has indeed been quite clearly different since long ago when everyone knew that Bitcoin had a supply limit as you said. While fiat currency guaranteed by the state is clearly not stable when world conditions are increasingly chaotic, either because of war or because of the increase in the price of goods that are always needed by many people. So we all have to be able to choose the best option by making fiat work in order to bring more stable profits for ourselves so that we can buy more Bitcoin continuously by utilizing the profits we get through fiat.