It is not the safe haven asset scarcity. People who want to buy safe haven assets always have the option of buying Tbills or gold.
Quantitative easing and ample liquidity has resulted in money flowing into different asset classes, including Bitcoin. This is different from safe haven asset scarcity.
To some extent it is the safe haven asset scarcity which makes Bitcoin one of these asset classes which people put their money into. Also, since the amount of new Bitcoin that's introduced is set and can't change, that makes it a better system for scarcity than gold, which could be mined more or less often at any time. Quantitative easing is just a factor which leads people to turn to assets or currencies which have the feature of save haven scarcity.