Answer
Because one has already performed sinful activities, those karmic reactions are stored as accumulated karma. These are like huge bags of karma carried from countless lifetimes. To exhaust all of them through the normal material process would require passing through the cycle of 84 lakh species of life — taking birth again and again in different species.
You may ask: if animals are not creating new sinful reactions, then why do they continue taking birth in those forms?
The answer is that this present human life is only one small chapter in a journey spanning millions of births. We see ants being born and dying within days, mosquitoes living only briefly, while human beings may live for a hundred years. This shows that time is relative within material existence.
Out of the 84 lakh species, about 4 lakh human species are considered human forms. It is especially in the human form of life that one performs conscious actions and creates fresh karma — both pious and sinful. The reactions generated here must later be experienced.
When the soul descends into animal forms, those births are generally not for creating new karma. Animals mostly act under the force of instinct and nature. They are not held responsible in the same way as human beings. Rather, they are undergoing the reactions of previously accumulated karma created during human life.
So animals are not creating fresh papa karma; they are simply exhausting old karma. That is why human birth is so rare and valuable. It is the opportunity not only to stop creating further karmic bondage, but to completely destroy accumulated karma through Krishna consciousness, devotional service, and sincere chanting of the holy name.
