"They" had been steeped in the BS of gold standards and the concept that the currency-issuing government must "get" money from some mysterious outside source before spending or that spending would be automatically inflationary.

Democracies don't generally elect people who are adept at the mundane task of running the government, and economists at the time were all educated when Keynes was considered radical. They also get their paychecks from the central bank or some entity representing the interests of the central bank and "markets".

As soon as the government took on debt in a currency it could not produce on demand (US dollars) it effectively gave up a part of its sovereignty. The US was mass producing US dollars and flinging them about the world to anyone who was dumb enough to accept their debt, so what difference was there between that and simply creating more money in-house by spending it into existence for the public purpose?

Thatcher and RayGun couldn't have been more aligned if they were joined at the hip. By simply deluding the people into believing that money grows on rich investors and not their governments they put the disaster of trickle-down econ in motion and the results of that show for themselves after almost a half-century of decline in both economies.