Because the manufacturer recommendations are based on achieving the best outcome for the individual receiving the treatment. But we're in a global pandemic, a situation the world has not faced in living memory. A different quality metric might be appropriate under these circumstances.
So even though the manufacturers (the companies that developed the vaccines, the scientists, the people running the trials, the ones that know the data and went through a full year of tests) keep on recommending 2 doses and the advice given to the UK government was still to stick to the 2 doses, we're now going the way that "we think we know better and it's better to try something new and untested because it's a pandemic and it might be better to have less efficiency to more people, than full efficiency to less people", even though it's, again, not tested and a shot in the dark?
