My guess is that they started with buyers because buyers typically are less wedded to an agent. Most agents start out on the buy side because significant numbers of buyers have no loyalties, and because buyers "pay nothing" for the services of an agent (as opposed to the sellers, who pay 5-6%), so customer acquisition is easier. (Of course, buyers do end up paying, it is just wrapped into a 30 year monthly payment.)

All this only applies to the US market,not sure about international RE. Source: I worked for a brokerage for years.