Because of the limited supply of Bitcoin makes it even more preferable to be used for world reserve as it would become very scarce and when something is scarce it is then valuable.
Bitcoins is limited at a total supply of 21 million. And so far, More than 18 million has been minted so far. We only have a few left to be mined, Do you actually think does people holding Bitcoin would want to sell theirs?..
Scarcity does not necessarily make something more valuable, that is wrong, without demand and supply then Bitcoin's price would not even move.