It's a narrative device.

Clearly Oppenheimer's security clearance is an issue and the writers and the director want to show what happened with regard to the clearance and whether Groves would clear him knowing what he now knows.

In fact, much later in the movie, Groves does answer the question.

Under my interpretation of the Atomic Energy Act, which did not exist when I hired Dr Oppenheimer in 1942... I would not clear Dr Oppenheimer today if I were a member of the commission.

It's a narrative device.

Clearly Oppenheimer's security clearance is an issue and the writers and the director want to show what happened with regard to the clearance and whether Groves would clear him knowing what he now knows.

In fact, much later in the movie, [2hr 38m] Groves does answer the question.

Under my interpretation of the Atomic Energy Act, which did not exist when I hired Dr Oppenheimer in 1942... I would not clear Dr Oppenheimer today if I were a member of the commission.