In Treta Yuga, organization of society was required, because people had more material desires. Not everyone was committed directly to full spiritual involvement, and therefore it was required that there be some production. They required some more opulent food stuffs; then they have to be produced and gold and jewels—these things were available but some endeavor was required to bring them out. So then society needs organization. Then varnasrama began: brahmanas, ksatriyas, vaisyas, sudras, each with their specific mentality.
Lecture on SB 3.6.32 – About the Four Varnas – Vyasa Puja 2008 – 5th Jan'08 – Tirupati