Why does Lord Vishnu always take incarnations in the upper castes only?

Why does Lord Vishnu always take incarnations in the upper castes only? Not True. Only four incarnations of Lord Vishnu are said to have been born in upper castes. ( The system which exist only in material world and if you consider it on the basis of birth) Lord Vamana (Brahmin) Lord Parsurama (Brahmin) Lord Rama (Kshatriya) Lord Buddha (Kshatriya) Lord Krishna, on the other hand, who is Paramavatar of Bhagwan is casteless (again if you take it on the basis of birth) because he is birthless. Being unborn, eternal Krishna is beyond any kind of classification. He is class apart. Further, God has not fixed number of incarnations.. The number of his incarnation is innumerable. O brāhmaṇas, the incarnations of the Lord are innumerable, like rivulets flowing from inexhaustible sources of water. (Bhagavatam 1.3.26) Some incarnations got immense popularity so the author of Puranas enlisted them in a set. That's it. So Do you really think in these innumerable incarnations God had only thought to descend as dvija. ( privileged class) You talk about human, what human He appeard even in animals, and how can you forget his third avatar which is of a Boar. (In the sequence of ten incarnations). I hope you know the fact that Boar is usually considered as lowest in animals. And you say God incarnates only in upper castes.He too discriminates. NO……..!!!! For God everyone is equal. He himself states so. I am equally disposed to all living beings; I am neither inimical nor partial to anyone. But the devotees who worship Me with love reside in Me and I reside in them. (Gita 9.29) Don't you believe on God's words ?? .. Thankyou. Edit : Balrama 19th incarnation of Lord ( In the sequence of 22 avatars - Bhagavat Purana) is not from upper caste. He belonged to yadav clan the the community mainly engaged in animal husbandry & farming at that time. Thankyou. Jai Sri Hari…!!!