why is the rapture theory so popular amongst Protestants if it’s a fairly new theology?
The rapture isn’t new, many church fathers believed it. I think St. Irenaeus would be the most popular on the subject.
Irenaeus (130 A.D. – 202) in Against Heresies 5.29:
“And therefore, when in the end the Church shall be suddenly caught up from this, it is said, “There shall be tribulation such as has not been since the beginning, neither shall be.”(2) For this is the last contest of the righteous, in which, when they overcome they are crowned with incorruption.””
beyond that its not really a question of how old it is, but is there a basis in scripture.