anime, I believe they timed it close to when Kawahiro helped the family to emphasize the point. The characters known to be the strongest illusionists and/or most mysterious at that point were Kawahiro, Flan, Mammon, and Mukuro. Since Mammon was dead, we should assume he was not included. The same goes for Daemon Spade: since he's dead, he's an afterthought. Therefore, Mukuro, Flan, and Kawahiro were left from which to choose.