He has the authority not to be intimidated, the sophistication to home in on substance, and prestige enough not to be in need of the publicity he might garner from a reportorial ambush. Lehrer had all these qualities in spades.
Most debate moderators pose pointed questions.
Lehrer took another approach. Rather than posing particular questions that required specific answers, he preferred to lead the discussion by introducing general topics. He did sometimes follow up and pursue a topic in more detail, but seldom in a way that really pressed the candidates or made them pay a price for repeated evasions or lack of candor. Consider, for instance, the way Lehrer introduced the discussion of foreign policy in the second debate, in Winston-Salem, North Carolina. These were opening gambits, and Lehrer continued with questions about how to end the violence in the Middle East, whether President Clinton was right to go to war in the Balkans, and others.