It's all because schools generally focus on the common denominator and more talented children are lost to boredom and lack of challenges. This doesn't apply everywhere, but it certainly applied to my pre-university education.
I skipped a grade, but even that wasn't enough. Parents indicated later that there was thought of skipping 2 grades, but the social/age difference might have been too great, so it was nixed at the time, then never revisited.
