Single player games franchises are frequently steeped in ongoing lore and character development. It's like following a major book or film franchise. Each new release is like a new episode, filling things in. While gameplay is important to a single player game's success, a mediocre single player game can still often succeed if it's part of a strong IP with a good ongoing story that lures people in.
Conversely, multiplayer games are often unfocused on story. They're about having fun. There's no character development beyond grinding out levels to gain more skills, weapons, cosmetics, loadouts, etc. The gameplay is all. A mediocre multiplayer game typically won't last very long because there are so many alternatives out there offering very similar experiences. Whereas the story and characters and worlds in single player games are investments on the parts of the player. The new game comes out in a series, and there is much more of an "obligation" to buy said game and play through its story.
Some franchises straddle the line by having a good single player and multiplayer component, but this is often more the exception than the rule.
In short, no one is emotionally invested in mutiplayer live service looter shooter number 916. It will sink or swim based on its gameplay, value, player engagement, and reliability (as in, you can find matches and the servers aren't shit). People are far more invested in the next chapter of a continuing story.