Tyler Cowen doesn’t so much address the question as assert its truth and offer six possible mechanisms. I would add two:
7. Influential people’s opinions become conventional; thanks to the fact that he has a New York Times column “what Paul Krugman thinks about economic policy” largely just is conventional wisdom among American liberals.
8. An ethic of responsibility—the more influential you are, the more risk there is that someone will take your “interesting” idea and actually put it into practice, with potentially disastrous results. The influential person sticks to those among his views that he has the most confidence in, which are probably his most conventional views.
Alternatively, maybe we just perceive as “influential” whoever’s opinions seem most conventional.