Because too many people were getting A*s on their GCSEs so they had to make it harder to balance it out again. The reasons why so many people were getting A*s is debatable but yeah if they didn't do anything then it would eventually be that there's no point in having high grades as they're too easy to achieve. They also wanted to further differentiate more able candidates. Like someone who got a 9 would previously have just got an A* whereas having the grade 9 shows that they're even better