The correct answer to this open question is the following.
Although there are no options attached, we can say the following.
What weakened Harald Hardrada's claim to the throne was that he had lived 15 years in exile. During this time he was a mercenary, looking for profits fighting battles that did not belong to him. He insisted on claiming the throne of Denmark and England on the fact that King Knut, his ancestor, had been the King of England in 1016.
He finally could get to the D*nish throne until 1064, and two years later, to the throne of Britain. This was a time when the Viking's presence in this region of western Europe was total and permanent. Vikings were known to be fierce warriors that destroyed everything and took possession of many lands at the time.