It's simple:
Because competitive multiplayer evokes competitiveness in players,and that competitiveness that makes players want to be better than others,make it easier for the publisher to push on DLCs.
A single person might work just a day and make an aesthetic dlc like a wearable hat,and then they will charge this DLC for 1$. In multiplayer,many players will by the hat to make them look nicer than other players. But such a DLC would be irrelevant in single player. Multiplayer opens a market for publishers to make more money out of each game sold,by taking advantage of people's will to look cooler and better than other players on multiplayer environments.
A single player DLC should at least feature 3 hours of gameplay. That means that the developers would have to work on making new levels,perhaps new enemies and guns,and also perhaps a new costume.
But if the game has multiplayer,the publisher can release the maps alone as a DLC,the guns alone as a "gun pack",and the costume alone too,and thus make up 3x times more money.
The thing they don't get is that there are many people buying some games just for the single player,and that gradually as more focus is turned on the multiplayer,the single player's quality will be declining,and eventually the games will start to make less sales overall,which in turn will impact also the sales of DLCs.
These kind of publishers (like EA) are thinking more of a quick buck than a stable future,and mark my words that this will hurt them sooner or later.