the power struggles which saw France and Spain continually plotting for power over the English throne were a huge factor, as was the somewhat dizzying to-ing and fro-ing of certain 16th and 17th century regimes, back and forth between Catholicism and Protestantism, which thoroughly de-stabilised the government for decades. Banning Catholics was an expedient and effective measure, suited to the times.

Of course it's now an outdated piece of legislation, but the
Monarchy itself is an equally outdated irrelevance. Judging the past by contemporary standards inevitably leads to silly and wrong-headed conclusions.