Because she was a Catholic, while the rest were Protestant. The Protestants “won” the issue over whether England would be Protestant and Catholic, and so the Catholic queen was consigned to history as a bloody tyrant. Bartholomew Traheron, an English Protestant who went into exile during the first year of her reign, wrote that she “swimmeth in the holy blood of most innocent, virtuous, and excellent personages” (by which he meant Protestants- remember that the Protestants carried out similar atrocities against Catholics), and this was a sentiment that was widespread among English Protestants.

Arguably the biggest reason for Mary I’s infamy, though, is a man named John Foxe. He authored a book, called Foxe’s Book of Martyrs that claimed to accurately detail the persecution of Protestants under Catholicism in England- it is worth noting that the accuracy of his work is dubious at best. His book was read throughout England, and helped cement the idea in the minds of Protestants that Catholic monarchs, ESPECIALLY Mary I, were brutal oppressive tyrants who attacked the true faith, while never mentioning the equally heinous crimes committed by Protestant monarchs, namely Mary’s father and siblings.

Some people also claim that sexism played a large role in the demonization of Mary. I personally don’t find this plausible, considering that her younger sister executed far more people under her reign, yet received no monicker like Mary did. However, it is impossible to say for sure whether or not this was a factor