Given that it was Rishi's seat, I suspect the electoral staff knew that it would have live cameras on him, so thought it might be less disruptive to ignore him rather than interrupt the speech. A lot of media feeds I saw looked as if they went in close to try and crop out the sign.
Not a Rishi fan, but it pissed me off as it's hugely disrespectful to all of the candidates, the staff there and most importantly all of the constituents that have voted. The least he could have done was stand still and behave for 20 minutes.