What a slap in the face.

Mike Vrabel, linebackers coach for the Houston Texans, turned down an offer to become the defensive coordinator for the San Francisco 49ers on Thursday. This job would have been a promotion for Vrabel, and he was the Niners’ first choice. He still didn’t want to work for them.

What a knock on the franchise, the roster, the front office and the head coach, Chip Kelly.

Who knows why Kelly even wanted Vrabel in the first place? Making Vrabel the top choice was Kelly’s first mistake. Vrabel is a former player who has been a linebackers coach for only three years. Never has he been a defensive coordinator. He’s a beginner, someone who needs a mentor.

Kelly needs an experienced defensive coordinator who knows what he’s doing. Someone who can run an entire defense by himself. Kelly can’t run a defense — he’s an offensive coach. In Philadelphia, his Eagles ranked 29th in total defense in 2013, 28th in 2014 and 30th in 2015.

For some reason, Kelly didn’t want an experienced defensive coordinator. He wanted an experiment.

We’re not in science class, Chip.

We may never know why Kelly wanted Vrabel, but we probably can guess why Vrabel didn’t want the Niners. Here are four logical reasons.

Logical reason No. 1. Houston’s defense isn’t on the field all the time. The Texans keep their defense fresh by using an NFL offense that values ball control and time of possession.

Kelly doesn’t value either of those things. He uses a no-huddle offense all game no matter the situation, and every week his defense is on the field for about two-thirds of the time. Naturally, Kelly’s defense wears down as the season progresses. This happened each season in Philadelphia.

Why would Vrabel want to coach a defense he knows will wear down, knows will give up a ton of yards and points? Why would Vrabel want that on his resume? Being Kelly’s defensive coordinator is no way to get ahead in the NFL.

Logical reason No. 2. Houston has better players. Put it this way — would you rather coach Jadeveon Clowney or Corey Lemonier? One is a lemon, and the other is one of a few excellent players on a team that ranked third in total defense last season.

The Niners defense ranked 29th in 2015. Its best player was NaVorro Bowman, a tremendous competitor who moves like a 35-year-old after returning from a devastating knee injury he suffered in 2014. Who knows how long his career will last?

Besides Bowman, the Niners have young defensive players who have accomplished nothing in the NFL. Guys like Arik Armstead and Quinton Dial.

Who would you rather coach, Dial or J.J. Watt?

Logical reason No. 3. Houston has stronger ties to Bill Belichick. Kelly probably thought he could get Vrabel because they’re both tight with Belichick. That’s the connection. Vrabel played for Belichick, and Kelly visits Belichick on occasion. They’re friendly.

Maybe Kelly thought Belichick would put in a good word for him and advise Vrabel to take the 49ers’ job.

Forget that. Belichick’s ties are to Houston. The Texans are Belichick’s extended coaching tree. Head coach Bill O’Brien and defensive coordinator Romeo Crennel both coached for Belichick in New England. Those coaches, including Vrabel, affect Belichick’s legacy. It’s in his interest for Houston to succeed. Belichick has no reason to care about Kelly’s 49ers.

My guess: Belichick advised Vrabel to say in Houston. Crennel has only one year left on his deal, so Vrabel can replace him as defensive coordinator in 2017.

Logical reason No. 4. Houston probably has more money to pay coaches. I have no idea how much money the Niners offered Vrabel, but I doubt they offered him much.

The Niners probably don’t want expensive assistant coaches. Think of all the money San Francisco owes former coaches who still are on the street without jobs. Guys like Geep Chryst and Eric Mangini.

And think of the $10.5 million the 49ers still owe Jim Tomsula. They’re not in position to pay a coordinator top dollar. Vrabel probably realized that.

My guess: Vrabel never had any interest in the Niners. He used them to pump up his leverage with the Texans and get a raise.

What a slap in the face.

Grant Cohn writes sports columns and the “Inside the 49ers” blog for The Press Democrat’s website. You can reach him at grantcohn@gmail.com.