Same reason the English forced their language on the Irish. To demonstrate dominance over a defeated rival.
That's really not why the Irish language almost died out. The English didn't view the Irish as a 'defeated rival,' for a start. The various attempts at stamping out the Irish language- never as dangerous as the broader economic dominance of English by the nineteenth century- were aimed at cementing the Irish within the British state.
The same for the attempted destruction of Te Reo Maori, and the hideous machinery that was English-language 'native schooling' in Australia, Canada (and I believe the USA.)
In
none of those cases was English cemented as a... as a piece of dick-waving. It was part and parcel of the colonial project.
The Americans aren't trying to do that in Germany! They don't want to be in Germany in a decade, let alone a century! Why would they embark on a hugely expensive and unpopular project with no chance of success, no support from their allies, the active opposition of the Soviets (who as has been noted will be astonished at the easy propaganda victory they've been handling) all to let the Germans know they've been beaten?
Why?
Do you really think there was anyone in US command saying 'Well, Ivan got to the Reichstag- and we've put their old government on trial- and we've got troops patrolling their streets- and those streets are full of rubble after the bombing campaign they were helpless to stop- and there's no German government in existence right now- and we control all the press. I dunno if they've got the message, what can we do to 'demonstrate dominance?'