Because these are not especially sober times. None of the candidates has to get us out of Vietnam or a Depression. In my youth, I remember Adlai Stevenson was considered quite a wit. He lost badly to a witless Ike Eisenhower. And funny JFK almost lost to the most sober, hangdog-looking politician of them all, Richard Nixon.
Few politicians can ever hope to top that.
Though a lot of them could get laughs right now with the same line.