Why should WE suffer in this birth for our karmas of previous births?

Some karma can not be given reaction within the same birth.  

For example, if a boy spoils the life of many girls, he can not be given punishment within this birth.  He has to take birth as girl again and be spoiled by many boys in his next birth.  So, he has to take birth as a girl.

If a boy tortures his wife in this birth, he has to take birth as a girl again and be tortured by her husband.  So, he has to take birth again.

This is just an example.  Think like this.  You will get the reason.

Some karmas require another birth to clear them.

Therefore, unless we completely desert our material attachments, we can not stop adding and stopping the reactions of karma.