There was little selection selecting against the loss it when it was abundant in out ancestors diet at that particular time and place. so by the time it became

There was little selection selecting against the loss it when it was abundant in out ancestors diet at that particular time and place. so by the time it became an issue it was already missing from the gene pool and as it is easier for a mutaion to break a gene than it is to gain function so its unlikely to mutate back into the genome by chance.