Sleeping Shaq - A S C E N D E D (For real, why didn't it go through Joseph's mind ? Araki Forgot. Or he simply didn't want to do anything with Hamon anymore)

And the biggest reason of all: they had no fucking time. Joseph became a master in 30 days, but only because he was extremely talented already and went through training that was even more intense than Jonathan's.
Jotaro would have to do everything that Joseph did and then some, while they were travelling half a world away.

He didn't want anything to do with Hamon anymore, since you can't really introduce the cool new way of fighting if you have the protagonist still sticking to the old ways.