The other players in a multiplayer game can often be classified as: a) assholes, b) racist assholes, c) racist sexist assholes, and d) so high right now. They feed the enemy team, shoot you in the back, spam the chat, play terrible music so loud it gets beamed into voice chat, and in general make you wish you were dead. They're not worth the pixels they take up on your screen… except, they actually are.
Players--not new characters, levels, or costumes--are the most valuable content a multiplayer game can provide you. Without other players, all of the other stuff is pointless. The easiest way to make sure that high-value players stick around for a long time is to give them plenty of company, and the easiest way to do that is to bulldoze one of the largest barriers to entry: the price tag.