The reason why multiple financial institutions outside the US are rejecting US citizens is called FATCA - Foreign Account Tax Compliance Act.

FATCA forces "Foreign Financial Institutions" to provide burdensome reports on every "US person" (US citizen/resident or green card holder) they happen to have an account with. If they don't do so, they suffer a 30% tax (or something close to it) on any financial instrument they might hold in the US. And I think it has some sort of recursive function too: to be compliant, you must ensure that every financial institution you work with is also compliant, or you risk having your financial instruments in the US heavily taxed just because you accept to do business with someone who did not respect FATCA. I don't know for sure what "do business" mean though. What I know is that due to US's weight in the financial world, the US government is able to impose an international law.

The easiest path for such FFIs is to simply deny doing business with "US persons". This way they're compliant.

This is the most outrageous form of capital controls ever enacted. It isn't an explicit capital control, but in practice that's what it is, since from now on US persons will have great difficulty to send money abroad. And it's outrageous because it's one government forcing financial institutions of the whole world to behave as they want. In many countries, for FFIs to comply with FATCA, they'd need to violate local laws.

FATCA is already responsible for a raise in the number of US citizens dropping their US citizenship. I think many more will do so in the next couple years.