Reasons Reasons
Tom Selleck interviewed with Closer Weekly. He revealed that he is enjoying working in Blue Bloods since it does not require him to come to set every day, unlike with Magnum P.I. All 163 episodes that they did, Selleck had to go to work because daily for the show. He added later on that he has intentions to leave Magnum P.I.

Reasons Reasons
Not The Only Reason
Further, in the interview, Selleck explained that leaving Magnum had a lot to do with his family. The show has been taking a lot of his time and affecting the work-life balance that he desired to have. Choosing his family over his career is a sacrifice that he is willing to take.

Not The Only Reason