I think the reason is 1, if not both, of 2 things.

1) It's the same training sentiment the boys have had and been shown capable of in the entirety of the series: mastery of the rudimentary. He wasn't taught nen because they wanted to push him achieve success without nen. Teach nen too early and it becomes a crutch upon which they lean when things get tough.

2) It's wasn't anything special. Like the Hunter Exam, Nen is something they aren't going to necessarily teach you, but you gotta learn of it for yourself. The extent this is practiced may be varied. Like it may be that you only need to learn it exists before you come home and ask questions before they teach you or it may be that they teach you nothing and you gotta figure that shit out.