Yes....the payday and the allure of being a star. To Khan you face Brook you are King at home, face Canelo you are a star. He knew he wasn't beating Floyd or Pac but they are all he campaigned for. Now Canelo fell in his lap, the literal nightmare for him and because he would be on big stage he jumped at this. If he fought Brook and lost its over, he can lose to Canelo and still get fights because no one expected him to win like losing to Floyd.
He can face Canelo in May, due his Ramadan in June, and then come back and face either Danny Garcia (because he's still #1 and mandatory opponent for Garcia's WBC championship) OR face Brook in the UK for the IBF Championship at the end of the year.
It's actually a win-win for him. At very worst, Khan will possibly have faced either Canelo & Kell Brook or Canelo & Danny Garcia in 1 year...that's tough.