Simon Cowell’s decision to leave “American Idol” after this season proves, once again, that if there’s one thing he knows better than music is business. How can the No. 1 star walking away from TV’s No. 1 show be a good idea?

Here are a few reasons:

1. “Idol” is on its way down.
It’s been slipping in the ratings in the past three seasons and the exit of Paula Abdul will only make things worse. Yes, Ellen DeGeneres has a following, but I’m going to guess that most of it overlaps with existing “Idol” viewers. There really was a good balance in the judges’ personalities (before Kara DioGuardi was thrown into the mix) and that’s gone now. When Cowell leaves, things will only get worse.

2. Being an owner always beats being an employee.
No matter how well compensated Cowell was – and could have been – on “Idol,” the upside for him on “The X Factor,” which he owns, will be much bigger, especially if he grows it from a start-up to an “Idol”- challenging hit.

3. The “Idol” audience isn’t the music industry’s audience any more.
With the new releases from last season’s winner Kris Allen and runner-up Adam Lambert underperforming compared to previous “Idol” finalists, it’s becoming pretty clear that more and more people who watch the show weekly have no interest in buying music, even from those who they voted for week after week.

4. Susan Boyle. The stunning success of SuBo shows that Cowell really does know how to give people what they want, even when it comes in unexpected packages. Of course, Cowell generally only gets involved in building careers when he gets to control everything – something that doesn’t happen with “Idol” winners, but does happen with certain “X Factor” winners like Leona Lewis.

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5. Boredom. Toward the end of last season, Cowell seemed like he was alternately bored by the process or agitated by the growing number of production issues on the show. Dealing with a new set of challenges will likely energize him.

PHOTO: Simon Cowell by Associated Press.