The theory generally goes that what convinces people to use the USD is that it’s backed by the full faith (and more importantly, full military force) of the US government. The Breton Woods Agreement and after getting off the gold standard the “petro-dollar” solidified the USD as the reserve currency for nearly the past 100 years.

The flip side is that unfortunately the stability of the USD is in the hands of politicians and other stewards of the country, which is an immense power to wield, and highly susceptible to corruption and misaligned incentives.

The idea behind cryptocurrencies like Bitcoin is that the code and protocol are simply open source computer code. China can’t arbitrarily decide that they have more bitcoins than they actually have, even if they dedicate an enormous amount of power and influence to try to do so. It provides a tool by which people can transfer value even with hostile adversaries. There’s also a fixed supply that will ever exist, and while deflationary money clearly has issues when it’s the only currency used, as a counterbalance to politically-driven inflationary money it seems like a useful hedge, even more so than gold (we have estimations for how much gold exists but we can’t ever say with certainty - but you can search literally the entire galaxy for more bitcoins and you won’t find them).