http://www.ewtn.com/library/ANSWERS/HOW2PURG.htm
Because the doctrine of purgatory was held by pre-Christian Jews, post-Christian Jews, Catholics, and Eastern Orthodox, nobody thought of denying it until the Protestant Reformation, and thus only Protestants deny it today.
There’s no Scripture to support it.
It’s irrelevant how many people believed it for how long.
It’s wrong and right and wrong do not change in God’s economy.
Without the shedding of blood, there is no remission of sins.