It was extensively trialled, brought in over a sensible time frame and there was plenty of training and consultation.
I understand that some DC even sat both GCSEs and O-levels so that standards could be carefully compared. That would have been hugely helpful in this situation.
Anyone who hasn't been paying close attention may have forgotten the sample assessment material farce of 2015. Many schools had already started teaching the new maths GCSE when AQA decided to threaten to sue the other exam boards because their sample exam papers seemed easier than AQA and AQA thought they were stealing their custom. Ofqual had to step in and got a bunch of students to sit the sample papers and what they found was that all but I think one paper was too difficult, grade boundaries would be on the floor, and that none of them were fit for purpose. The exam boards had to scrap their papers and go through accreditation again. Any maths teacher could have told you the papers were too hard.
The thing is, the sample papers and the qualifications had been signed off by Ofqual without a single student sitting the papers. Once the papers were actually sat, the problems were obvious. The only reason this was picked up was because AQA threatened to throw its toys out of the pram.
Incidentally, exam board textbooks were written based on the original too-hard exams and are now sat in warehouses waiting to be pulped.