There are two factors. First, weight of numbers. Statistically, the proportion of the population attending private schools is quite small - hence you would expect a small proportion of representative sportmen to come through these schools. The second answer is that the overall standard of private school cricket is modest. Young players with talent have become as good as they can get at that level by Year 9 or Year 10. They then stand still for two or three years, instead of being toughened up in Grade cricket, like their counterparts in State schools. This of course doesn't occur in Rugby, since very few players are ready for senior club Rugby before leaving school.