I suspect a lot of this is media hubris: Journalists expect a candidate for political office to be up to speed on the agenda they set. "Crazy idea: When running for president, occasionally look at a newspaper," political journalist Alec MacGillis tweeted. Questions often test a candidate's ability to keep up with news coverage, and a passing grade is given for answers in line with the media orthodoxy on the matter. This rule is captured by a spur-of-the-moment cartoon in which Johnson asks, "Who does know what Aleppo is?" and the grimy Syrian boy rescued from the city's rubble, known to millions from an iconic news photo, raises his hand. The implication is, you're not qualified to be U.S. president if you haven't seen the image and internalized the narrative that went with it.
It could be argued that being versed in these narratives might actually be an impediment to a top office-holder. I've actually been to Aleppo, and I have no idea what it's like today. The numerous warring factions' views on the drama unfolding in what used to be Syria's biggest city and commercial center differ greatly, and even to someone with extensive contacts in the area, it's difficult to get a bird's-eye view. I would fear Johnson if he had a competent-sounding answer to the question of what he'd do about Aleppo. People on the ground and current leaders receiving information from lots of different sources have spent years looking for solutions and haven't found good ones, and the little boy's memetic image is proof of that.