My country did the same. The answer is simple. Education research. Being one year behind isn't a big deal (and use not to be), but having a few 10-11 yo in the same class as 8 yo was detrimental to everyone. We then created special classes for people with learning disabilities, which is still detrimental to those kids, but at least the impact is limited.
Academic prowess shouldn't be such a social booster/crusher, especially pre-PhD, but it is, so we have to deal with it, and that mean not making kids repeat classes too much (two decade ago, it was max a year below 11, max two after that in my country, nowadays it's just avoided as much as possible).
Academic prowess shouldn't be such a social booster/crusher, especially pre-PhD, but it is, so we have to deal with it, and that mean not making kids repeat classes too much (two decade ago, it was max a year below 11, max two after that in my country, nowadays it's just avoided as much as possible).