I think most of us have had fun playing a game online with others recently. Multiplayer is something that's become pretty mainstream since the early to mid-2000s with things like XBox Live, Battle.net, Steam, etc. Some games are now entirely multiplayer-based, and many prominent games, in addition to their single player campaigns, provide great multiplayer experiences.
So clearly multiplayer is this great thing, and every game should have it then... right? Right?! If you read the title of this article, you know the argument I'm making is... no. I'm sure there are many reasons this is the case (cost, not every game works as a multiplayer game, etc.), but I'm going to focus on one: network effects.
Let's say there's a game that's your favorite. You're having a blast playing it, and you want to play it with some other people. You find out it has multiplayer - awesome! You go to browse for a game, and...
If you can't find anybody to play the game with, exactly how valuable is the multiplayer feature? I'm going to go out on a limb and say not very. Going back to network effects, the idea behind it (phrased so eloquently on Wikipedia) is that the "value of a product or service is dependent on the number of others using it". If lots of others are playing a game's multiplayer - great! By jumping in and playing too, you're likely to have a great time with it. If there aren't a lot of others playing...
You're likely to not have a great time - thereby taking yourself out of the pool of players playing, and reducing the value of the game's multiplayer for everybody else. This will put the game's multiplayer in a downward spiral which is difficult to recover from. Once this sets in, it's difficult for a game to recover.
If the game is great, why wouldn't people be playing it online? There are a lot of reasons. Maybe its big multiplayer heyday has ended. Maybe it's a smaller game that doesn't have the marketing budget to get enough people into it to reach a critical mass. Maybe the servers have shut down for the game making any sort of multiplayer possible.
I think the most important reason, however, is this: competition. The multiplayer competitive landscape is more competitive than ever. A lot of games now have business models built around multiplayer: skins, subscriptions, expansion DLC, etc. If people aren't playing those games, they're not making money. Therefore, they're going to aggressively market their games and take up as much multiplayer real estate as possible. This makes it difficult, if not impossible, for multiplayer to be successful in a smaller game.
We tried to include multiplayer in Starfall as one of our stretch goals. There's a reason it wasn't an early stretch goal, or even included in the base game: If the Kickstarter wasn't successful enough to reach that goal, then likely the game wouldn't have enough players playing it for multiplayer to be a success. Again, it goes back to network effects: there's not much value in multiplayer if others aren't playing as well.
The bottom line here: multiplayer isn't for every game. If you have something with broad appeal that will be played by a lot of people, then go for it! But if your game is likely to be on the smaller side, you may want to think twice before investing a ton of money and development into it.