The explanation of this sphynx-like puzzle starts with Paul Volcker's abandonment of the road to sound money in 1985/6. The defining moment came when the then Fed Chief joined with President Reagan's new Treasury Secretary, James Baker, in a campaign to devalue the dollar. The so-called "Plaza Accord" of 1985 launched the offensive.
Volcker, the once notorious devaluation warrior of the Nixon Administration (as its Treasury under-secretary), never changed his spots, seeing large US trade deficits as dangerous. The alternative diagnosis — that in the early mid-1980s these were a transitory counterpart to increased US economic dynamism and a resurgent global demand for a now apparently hard dollar — just did not register with this top official.
Hence the opportunity to restore sound money. But this comes very rarely in history — only in fact, where high inflation has induced general political revulsion (as for example after the Civil War) — was inflation snuffed out. In the European context this meant the end of the brief hard-Deutsche-mark (DM) era and the birth of the soft euro.