Being declared illegitimate when younger would make some people consider her claim to the throne as weak, despite being declared legitimate now. This could lead to rebellions and civil war. Elizabeth's brother also over ruled the third act of succession, making Lady Jane Grey queen instead of one of his half-sisters, which could be interpreted as him denying Elizabeth's legitimacy because even though he wanted a Protestant queen, and Elizabeth was Protestant and was going to be queen after Mary, he chose Lady Jane Grey