I think the real reason is because after Part 2 a lot happened and restricted Joseph from focusing solely on Hamon training, for example his daughter Holly, wife Suzie and his job. He could barely go and visit Jotaro because of Suzie etc. he had to push it off as something else.

Not only how restricted he was but also because the time of learning hamon was traumatising, he lost his best friend, watched his arm get cut off, witnessed and fought the ultimate being. He would rather put that behind himself.

Another point is that Dio could stop time and Joseph can’t do much against someone who could stop time, plus he lost that edge in battle to anticipate an opponent and to be cunning.

If Joseph were to defend against Dio well, then Jotaro would be moved from the spotlight. Joseph won by some ridiculous odds against Kars in Part 2.