The reason was that the succession had been set by Henry VIII prior to his death. Although previous acts had removed his daughters from the succession on the grounds that they were illegitimate, the Third Succession Act of 1543 returned them to the line of succession, after their brother Edward. This act was passed by Parliament, and only could be repealed by a subsequent act of Parliament. In 1547, The Treason Act made it high treason to attempt to undermine the Third Act of Succession.

In 1553, a 15 year old Edward VI was dying, and he and his councilors were extremely worried about Mary succeeding him and restoring the Catholic faith. Edward VI then wrote the Device for Succession, which removed his sisters from the line of succession on the basis of illegitimacy and instead named his cousin Jane Grey, granddaughter of Henry VIII's younger sister Mary and Charles Brandon, his heir. Jane was a devout Protestant, but more importantly, she was married to Guilford Dudley, son of Edward's highly influential councilor Lord Northumberland. In the king's final days, despite the Act of Succession never being rescinded by Parliament, Northumberland threatened influential judges, councilors, and members of the nobility to agree to Edward VI's device, threatening to brand them as traitors if they did not. Thus, when Edward VI died, the Privy Council named Jane as heir.

However, Northumberland and his supporters had badly underestimated the support Mary would receive not only from Catholics that yearned for a restoration of the Church, but from many others who, while Protestant, were more concerned about the rule of law. Another consideration is that the Wars of the Roses aka the Cousin's War was only two generations past. A straight succession through siblings that was codified by an Act of Parliament was far cleaner than changing the line of succession to favor a cousin instead of still-living siblings, and would leave less opportunity for future conflict. People generally wanted a settled and clear succession, not a changeable one that could land them right back to Civil War, and this was prioritized over religious concerns. Ultimately, seeing that public support for Mary was insurmountable, the Privy Council renounced Jane and crowned Mary instead. Northumberland, Jane, her father, and her husband were all eventually executed for treason for violating the Act of Succession.

This also tied Mary's hands in terms of Elizabeth being her successor. As she had used the Third Act of Succession as the basis for herself becoming Queen and executing her opposition as traitors for attempting to invoke Edward VI's device, she could not then disregard this very same act to disinherit Elizabeth, nor could she petition Parliament to repeal the Act, because doing so would invalidate her own claim as well. In addition to all of this, Mary desperately hoped for and believed she would have children of her own who would then carry on the succession, to the degree that she developed two phantom pregnancies, dying not long after the second one.