1.  The peace is the third reason.

2.  The third reason is that the developing countries's living level is improving little by little.

3.  The third reason is that the protect, and predict work is done better than the past years .

4.  The third reason is the medical condition got better and better.

5.  The third reason is that people has become more concerned about their living habits and patterns, which providing a more sanitary and comfortable enviorment for themselves.