Here’s Edward Wasserman in the Miami Herald:
But the equally compelling reason, and one that makes it unlikely that journalists will ever be warmly welcomed as political aspirants, has to do with the adversarialism that journalism is premised on. I don’t want to go overboard here… But fundamentally, to do their jobs right, they sign on to the idea they’re a check on power, an organ of accountability, working on behalf of the public and, when appropriate, working to expose or frustrate the stratagems, policies and privileges of office-holders in business and government.
A record of distinction in the newsroom is unlikely to win anyone a welcome among politicians, and it’s understandable that journalists move so rarely into public office. On balance, that’s a good thing. Ambition can be a motivator, but it can also be a powerful distraction. If you’re handling your current duties with an eye mainly to endearing yourself to your next boss, you probably don’t deserve either job.