I don't have proof, just circumstantial evidence, but the reason I believe she left was because she was the only cast member who did any work. She came to rehearsals and took her character's reactions and delivery seriously, while everyone else screwed around. The only person who supported her was Nicholas Colasanto, whom she had a good relationship with. Everyone else called her "difficult."

If you watch the show, watch Shelly Long when she's not talking. Her facial expressions constantly change to react to whoever is talking. In 6 seconds she'll literally go from smiling, to concerned, to angry, as a reaction to something Sam says. It's amazing.

In other scenes, Sam and Norm will be the focus of the scene on the far-right, front part of the bar where Norm sits. Shelly Long will be sitting on the far-left, back of the bar, barely in frame and out of focus on a stool reading a book and still give a goddamn reaction to what they are saying. It's amazing.

Everyone else mostly stands around waiting to deliver their lines.

No one worked as hard. Then they called her difficult. It was hell for her. You'd think they would have learned after she won an Emmy in season one for a show that ranked toward the bottom of the ratings, but I suspect that just made their reactions toward her worse.