Buddha didn't oppose Caste. The Charvaka Atheists mocked and vilified Castes in defense of Pleasure-Pain principle and atheism. They were loved so much by the Brahmins, they were allowed to give speeches in front of large lecture halls and in front of Princes. Yes, you read all that right. Then Buddha came and brought back Caste-defensive arguments and the Brahmins evidently really loved him a lot more and spent money building his temples. Most Buddhists were Kshatriya or Brahmin Caste in ancient times. It's also why Buddhism and Hinduism are okay with atheism.