He makes decent money, has an easy job to be successful at, and lives in a nice area. Unless you are motivated by money a lot i tis a great position to be in.
He's money driven.
He's "only" making $900K there (not sure what the new contract is). He took a paycut leaving Bama.
When he was at Bama he had remarked about his lifestyle taking a hit due to owing his ex wife a ton of money in the divorce.
I'm sure he lives comfortably, but if you're making $1 million/year and get offered $3.5 million to go coach somewhere else you're going to turn that down? Keep in mind, if he gets canned at FAU they probably owe him next to nothing (in the big scheme of "big time" college football). He goes to a Power 5 school and gets $3.5 million (or more) he likely has a huge buyout if he gets fired (Butters got $7.5 million).
Sure, FAU is a nice location....and lots of hot chicks. But Kiffin likes the spotlight and money. He gone.