I don't think it's that at all. I think it's just his personal pride and he can't face the thought of losing to Brook after all their back and forth over the years. He's always put Brook beneath him and slagged him rotten. Now I'm not saying Khan doesn't believe he can beat Brook, and Crawford for that matter, but I'm sure he knows deep down he might get beat and he might get KOd, and maybe brutally KOd. He's fine if that happens against Crawford or Canelo, but I think his grudge and his feud with Brook runs too deep and for too long for him to risk it. I don't have a side when it comes to Khan and Brook, so my neutral, honest opinion is Khan just really doesn't want to fight Brook. But I still think he might eventually, when there's no other options and it's his last resort for a big fight.