As far as I know, the following may be the reasons:
1. Good environment of school campus, open and big field;
2. Almost each kid in class is assigned with some responsibilities, e.g. class monitor, subject monitor, desk key holder...
3. The teacher will give coupon to encourage the kids who pay well attention in class, good performance in homework...
4. Almost every kid can choose at least one ECA, depending on the willing of the parent/kid.
5. In particular to the Chinese Lesson teaching through Mandarin.