The problem with multiplayer games is how much it costs to run them.
Look at this forum. Remember how the whole thing crashed several times right after VV3 came out? That's because the server that hosts the forum couldn't handle that many visitors.
I guess you could think of a multiplayer game as being similar to a forum. People have to login to the server, which hosts the multiplayer game, and that is how they connect to other players.
A multiplayer game requires many servers. One server couldn't hope to handle that kind of load.
Servers cost a lot of money both to purchase and to maintain. You can rent servers, but this generally is both expensive and unreliable. You are basically in the hands of another company, which owns the server, and if something goes wrong you can't do anything but wait for that company to handle it. To OWN a server you first need a place to put it. Those things are HUGE! Then there's the cost of purchasing one. A server costs several thousand dollars.
There's just no way this would be profitable for a small game company. Sometimes multiplayer games turn out to be either unprofitable or unmanageable even for huge game companies that can afford to take the loss.