Earlier quoted context omitted.
Because empirically, it doesn't work. The front page of google for [Boris Johnson bus] has stories about: - The failure of a busmaker - The brexit bus advert - The suspicion that his 'model bus' answer was this ploy.
It is likely that Google is surfacing the matches it empirically considers more relevant to you.
If he's trying to spike negative results for just a certain period of time, it's quite an effective tactic. News cycles quickly, so even if each instance is eventually caught, it may successfully push down recent news / current events until they're no longer current, which may result in millions of people missing the reporting.