Based heavily on my real-life circle of friends, so perhaps there is done bias to it, but here is how i see it. There are two big factors I've noticed.
First, in my experience, most people rarely delve deeply into any particular multiplayer mode. Being competitive enough to have fun in multiplayer mode takes more time and effort to do than messing sound for a few hours in a single-player setting. Given a random shooter's multiplayer mode and a modest-length single-player campaign, they get more time out of the single-player.
Second, whatever concept the game depicts that was the driving force behind the game purchase us usually sold to people more effectively through the context of a campaign than that of matches (when possible). Like, when people were first discovering halo, friends didn't pitch the game to each other in terms of what the scifi setting meant for battlefields and weapons, they did it in terms of "master chief is a badass." Story, setting, and thematic content at why the people I'm thinking of buy games to begun with. Dedicated multiplayer playtime is what happens when we have already been sold on game mechanics while in the prices of enjoying other things. Sure, you may reach a point where you roll with the setting and say "i can probably guess how a modern war game will play," but that can easily Co-inside with the point where you no longer care enough about the setting to try the game out. Prior buy games to go on adventures, not find a new sport to practice.
Tl;dr: even games that appear entirely gameplay and competition driven can still be at the mercy of narrative content to some extent.
