Why? Why did the series end?

[No need to know!]

It seems that Selleck was a bit tired and wanted to move on to a movie career. I have never seen any actual articles where it says that the ratings were so low that it was near a cancellation of that reason.
Larry Manetti have said in some interview that the show could have stuck around much longer.

Now the time just flies by, and i actually hope that we don´t see any movie or a new series. I am not interested to see other guys do these characters. I rather just watch the original show forever and ever.

[SignGuyHPW]

I think the network left it up to Tom Selleck as far as when to end the show. They'd have kept it on the air longer, but there were a few factors that I recall which lead to the end. The main one was Tom Selleck, himself, was wanting to move on to other projects. He was going to be able to make more money in the movies than on a TV show, but he probably had to turn down several roles due to the schedule on Magnum being a factor. If the main character is gone then it'd have been hard to keep going with it. He was such a major star at that time that there's little chance the viewers would've accepted someone else in Magnum's spot if they wrote in another character to take over for Thomas Magnum. Another factor was that it would've gotten progressively harder for the premice of the show to work. Say the show went 12 seasons. Now you've got three middle aged guys plus one that would've been near 70 running around as commandos several times per season. It'd be hard to make it work for the younger viewers which was a key demographic.

[Kevster]

I suspect that Tom Selleck's overall career goal was to become a top-tier movie star. I think Mr. Selleck really loved and appreciated his time on Magnum, but at some time, Mr. Selleck needed to use Magnum as a springboard into his larger career goal on the big screen. I recall that he attempted to make that transition after the first and second season of Magnum with High Road to China and Lassiter (I might've misspelled that movie) - and those two movies didn't stick. Perhaps Magnum may have been a much shorter run show had those two movies in the early eighties been bigger.

But I think Mr. Selleck really underestimated how important Mr. Hillerman, Mr. Mosley and Mr. Manetti were to his success on Magnum. Those other actors really helped bring a chemistry to Magnum that is not on other tv shows. Always seemed to me that Mr. Selleck seemed a little flat on the big screen (in my mind the exception being Quigley Down Under). And I think he seemed a bit flat because he was removed from that incredible chemistry on Magnum. No revelation that Selleck was the central piece to Magnum. Maybe the show might've survived if any of the other actors chose to depart, but it could not go on without Selleck.

About TS in other roles...

Like so many actors in so many parts, they have to find a way to give a character depth and/or make the character relatable. TS did that in spades with Magnum. He failed to do that in many other parts he's played over the years. If I had to narrow it down to one phrase that describes what made TS successful in a part, I'd have to use "multi-faceted personality." The best characters TS has ever played had a wide-range of emotions, and they really seemed to be legit in his layering of the character.

Many of the characters TS has played always seemed to be a little to flat or bland. When he took a character "less serious" or "less controlled," even if it was just for moments of screen time, it always seemed to enliven the character. That was a gift that TS used a little too sparingly, but rocked it when he used in Magnum, 3 Men, and somewhat in Quigley...

It's a shame he didn't find a couple of parts that were just a little more "right" for him and the times he was in. The guy had far more potential than what was realized. That may be one of the things that endears us to the Magnum character and the show. The guy was a deep well, and we knew that we might see that depth tapped at any moment.

My $.02.