The success of USD stablecoins vs non-USD stables is the reaping of seeds sown in the closing days of WWII.

Those in the know will recognize the Triffin Dilemma in the telling of the tale.
As the war came to a close, the US stood alone amongst the major economies. France, UK, USSR, Germany, Japan, China, and most of the world’s middling economies had been badly damaged by the war and the preceding Great Depression.
By 1945, almost the entire world needed rebuilding. That took money, unfortunately, and the Axis, Allied, and Communist Bloc countries had little left. Except the United States, who entered the war late and had been selling weapons and food to the Allies and USSR, often for gold, sometimes for credit.
Allied and former Axis countries they controlled took massive post-war loans from the US.
The USSR essentially took everything that wasn’t nailed down from conquered Axis nations occupied by the Red Army, supplemented with forced labor, to finance their rebuilding.
There was a problem brewing, however. The Fed was a vassal of the administration in those days, so interest rates were held down, allowing the government to inflate away much of its own wartime debt.
This made lending dollars to Europe especially attractive in a world where a long-dated Treasury bond yielded <2.5%.
So dollars were exported. The US was the major creditor nation in the world.
But the debtor nations eventually needed to pay those loans back. Still rekt, they didn’t have much to sell the US to get those dollars.
To help allies and lenders, the US airdropped USD onto Europe via the Marshall Plan.
Europe began to recover, its companies & govts able to service debt, and had goods to sell the US.
And they kept borrowing from the US, where the Fed kept rates low.
Dollars seemed to pile up in Europe. The USD had become the main reserve currency, since other major currencies were pegged to the dollar, it made it easy to settle in dollars for transactions in lira, pounds, francs, Deutschmarks.
In short, the dollar circulating supply grew!
This caused a lot of heartburn in the US.
Remember that the USD was pegged at $35/gold ounce. The supply of gold in Fort Knox was slowly shrinking as dollars were redeemed by foreign central banks, but the total supply of dollars outside the US was growing (remember that USD inside the US was not allowed to redeem, so could not cause a run).
Imagine a stablecoin issuer who had a large reserve, but over time the supply begins to approach or surpass your reserves. That was the US in the late 40s/early 50s.
Eventually the Fed stopped taking orders from Treasury in 1951, and they began to hike rates.
Now dollars (and all modern money) is manufactured by commercial banks. They take an asset worth $100 and loan $50 against it. The bank gets a $50 asset (the loan) and creates a $50 liability (bank deposit).
Nothing says this has to be done in America. So European banks began to manufacture USD deposits. Eventually an Asian equivalent in Singapore popped up. And Tether has a related model.
So how does this promote USD stablecoins today?
Because USD monetary policy is run for the benefit of the US economy, not the global economy.
That is to say, a glut or scarcity of USD can occur outside the US. When this is a scarcity, Eurodollars and Tether fill the gap.
Tether’s success has less to do with crypto and more to do with supplying USD liabilities where USD was in short supply.
Of course, the US avoided a run on Fort Knox by letting the Fed be independent and eventually abandoning the gold standard, since the Fed doesn’t really control the money supply very well, since they’re not the ones who manufacture them.
But no one ever solved the dilemma of mismatched USD supply and demand outside the US - which provides a tailwind to USD stablecoins