Too often (at least in the past 5 years or so), the publisher is afraid if the game is SP only that people will play it, finish it, then flip it back quickly and flood the market with used copies so that people will be more likely to buy it used instead of new (depriving the publisher of revenue). So they take resources away from crafting SP to make an often half-baked multiplayer mode to try and delay people from trading it back in.
Spec Ops the Line is the perfect example, the team finished the game, the publisher wanted a MP mode, so they took the development assets and gave them to another game developer to make a shitty multiplayer mode with no input from the original developer, that's why the multiplayer doesn't have any trophies because it is LITERALLY tacked on. Then because of that shitty, tacked on multiplayer it pulled down some of the review scores.