Illumi's only reasons for joining The Phantom Troupe are to look after Illumi, and to kill Hisoka, as the Magician paid him to. It's entirely possible that Illumi isn't even on Hisoka's radar, or ...

Chapter 377 of the manga reintroduces the full lineup of the new Phantom Troupe, but the interesting thing this time around is that Illumi Zoldyck has joined the Spiders as well.

Hisoka answered all the questions but slipped out his malice while saying he wanted to fight Netero. However, the latter didn't consider Hisoka to be a worthy enough opponent. 4) Illumi Zoldyck ...