A lot more can be said! But an oversimplified explanation:

Henry VIII broke off from the Catholic Church to start the Church of England. When he died, his three children pulled the country back and forth between the churches of Rome and England, resulting in many deaths, religious persecution, and terror. For the next hundred years or so, various monarchs also switched England between Catholicism and Protestantism, resulting in even more death and destruction.

Because the wars of religion had caused so much unrest, many people were eager to establish one region and stick to it rather than being jerked around. And Protestants especially feared the continental influence Catholicism would bring to England (particularly France, England’s longtime frenemy.) Eventually, the Glorious Revolution saw the Protestant Mary and William overthrow Mary’s Catholic father James II and officially change the laws of succession to exclude Catholics. It was the perfect storm of a tired and fearful nation, a fairly weak king, a strong usurper, and several political factors (mainly the Whigs) that combined and allowed for that huge change.

At this point, the rule is sort of a tradition. since the monarch is also the head of the Church of England, it wouldn’t make sense for a Catholic to be in line for the throne and potentially head a religion they don’t follow,