Liquidity matters. Cash is king because it's the most liquid asset. You can sell $100s million dollars worth USD/EUR/JPY/GBP in a day without affecting the price those currencies. Bitcoin is the polar opposite of liquidity. Bitcoin markets are notoriously thin. If you try to by or sell large amounts the price either collapses or skyrockets.

Bitcoin is like water tower where you can move water level (price) up or down by adding or removing 10-20 cubic meters of water. Fiat money is like a big lake or a sea. You can take or put 100 cubic meters per second and it's not visibly moving.

Once BTC price rises to the level that enough people feel rich and start buying things, the price will drop suddenly. Panic hits the markets and the price drops like a stone.