All games will have a certain popularity when their new. If developers want people to return to those games for year to come - add multi-player.
Daggerfall was a massive role playing game that was only single player. Who really talks about Daggerfall anymore?
Counter-Strike is designed around multiplayer. Who talks about Counter-strike, oh,,, tens of thousands of people.
However, doom and half-life are still talked about and played by who knows how many people and they were designed around a single player game.