There are a couple of reasons for this reality in chapter 9 of Orwell's work. On one hand, Napoleon being the only boar on the farm has sired all of the younger pigs. The ability to have power unchecked on the farm has also resulted in his own sexual satisfaction. The pigs born are now seen by Napoleon as his legacy, his insurance that his reign will continue on without any opposition. In this, he ensures that the young pigs are educated in the ways of Animalism, or more appropriately in the ways of how the pigs hold all of the political power. The fact that they younger pigs are kept apart from others and that other animals must step aside in deference to a pig upon encounter helps to advance the elitism that the pigs have established in both the social and political hierarchy of the farm. It is in this that there is a complete break with the original goals of Animalism and a replacement of Jones with the Pigs. This state of affairs will now continue for some time afterwards, as the education of the young pigs ensures that there will be future continuation of the Status Quo.
Napoleon thought that young pigs would be easier to manupilate and brainwash hence he decided to focus on educating the young.
They young pigs were educated so that they may follow in the footsteps of the leader. And there was created a sort of classicism to create division among the people, so that only a chosen set of people may be educated and the others un-educated donot leave their assigned work. The pigs were chosen as heirs to Napolean himself. They were kept away so that they learn only the ways of Napoleon and others may not influence their un-educated and simple minds. this was to confirm that the young pigs are on the same religion of the leader. the question why only pigs are selected is so that there is a unity in one class. and animals form other class may not hold a claim in the governmental affairs and know their designated place. Divide and rule!