Apart from the normal inclination of anyone not wanting to be perceived as an excessive drinker, it's possible Kavanaugh's answers were more calculated. Any acknowledgment of excessive drinking and drunkenness would help corroborate the accounts put forth by his accusers of what happened during his high school and college years. The suggestion that he was subject to blacking out could be used to show that he was in no position to know one way or the other what he had allegedly done to Ford. It could have undermined his claims of utter certainty that he never was at the gathering described by Ford.