As people have pointed out this means that the US or wherever can print money and controls your monetary policy, but that's actually the least of the problems. Euros and US Dollars in large quantities, and digitally transferrable versions of them, can essentially only exist as numbers in databases controlled by the US and EU government respectively. So by adopting them as your country's currency, you're effectively giving them control over your money. They get to decide whether it continues to exist, whether you get to access it, whether it gets transferred to some well-connected megacorporations in those countries that bribed your former leader into a dodgy deal... In short, it gives a whole bunch of control to powerful entities who have tended to use their powers in ways that have not been good for developing countries at all.

Because then you're beholden to another country, which may be hostile to your country, and their money printer.

With Bitcoin, no one has control!

If you are using another country's currency you are tied to their monetary and indirectly fiscal policy which most likely does not align to yours.

In fact trying to get a neutral or deflationary currency is a no brainer. Especially, if you have a small banking system/network.

Of course there are gotchas.

The US dollar was el Salvador's currency. They are trying to diversify away from that total dependence.

El Salvador already uses the US dollar, but it has no control over what the US does to its money supply.

Sure, but the ECB or Federal Reserve can print money, and you don't control them. If your goal is a mathematically guaranteed money supply (this would not be my goal, fwiw), BTC is superior.