She left because her five years were up and she decided to focus on her family and shifting into movies. Which she did. The need for her on the series had been mostly explored. And fortunately the writers and producers had such a deep bench of talent on and off the screen to just pick up and carry on.

I’ve been rewatching Cheers recently. Currently up to season three. She’s great in it but the series was definitely able to have solid episodes without the Sam/Diane tension. And yet use their history to move stories along. Some of my favorite episodes from that time barely feature her. But she’s also a huge part of many classic moments. They used the cast very well throughout the series.