Board Thread:General Discussion/@comment-98.168.186.146-20170817161327/@comment-28784495-20170904002619

I think because the Zoldycks, especially Illumi, wanted to keep Killua in this bubble, until they knew for a fact that he was completely brainwashed into thinking their way. Since Illumi was trusted with not having friends, along with Milluki and Kalluto (Kalluto does not have a "friendship" with the Phantom Troupe, like Killua has with Gon), they were allowed to learn nen. Killua on the other hand, was very hard to control during his time at the Zoldyck Estate, so maybe they didn't expose him to nen, since they wanted him to use it for assassination purposes only.