It's a matter of history. This is a very simplified version.

After Henry VIII broke away from the Roman Church, our next monarch was a protestant. He died young and there was an attempt to install his cousin, Lady Jane Grey, as Queen because she also was a protestant. Mary (Henry's first daughter) rightly claimed the crown so Jane only lasted 9 days as queen. Mary immediately tried to restore the Roman Church but so many protestants were put to death in her reign that she became known as Bloody Mary. Elizabeth took England back into protestantism (with quite a few more deaths [mainly Catholic but some protestant dissenters too]). Since Elizabeth had no heirs, the throne was offered to James I (James VI of Scotland) and we had the Stuart dynasty. James' son, Charles I, became so unpopular that there was civil war followed by a sort of republic or dictatorship. On the death of Cromwell, Charles II was made King. He promised parliament that he would reign as a protestant but was secretly a Catholic. His son, James II was also very unpopular (as well as being Catholic) so his son-in-law, William of Orange, was asked to take his place. William agreed and he and his wife Mary ruled jointly (the only time in English history as far as I know). By this time, rightly or wrongly, Catholicism had become so unpopular and was considered by many to be the cause of all England's ills. So the document quoted earlier (1701 Act of Settlement) was designed to prevent the sort of autocratic rule that the Stuarts had favoured. (I don't know about James I but the other Stuarts believed in "The Divine Right of Kings" which meant they could do just as they liked.)

I'm sure someone will deal with all my errors and omissions. This was just off the top of my head (and may be biased by my protestant upbringing).

Frank L.