One objection to sola Scriptura which we sometimes run across takes the form of a question: if sola Scriptura is true, then why aren’t more Christians Protestant? I suppose we could generalize the question by asking, If Protestantism is true, then why aren’t more Christians Protestant?
The underlying assumption is a direct correction between truth and popularity.
But in a fallen world, why would we expect truth to be popular? Indeed, in a fallen world, shouldn’t we expect falsehood to be popular?
For example, Biblical prophets are notoriously unpopular. Would it be reasonable to ask, If what Isaiah and Jeremiah said is true, then why didn’t more Israelites believe them?
Likewise, if what Jesus said is true, then why didn’t more Jews believe him?
For that matter, the Catholic objection can be easily turned right back on itself. If Humanae Vitae is true, then why do so many Catholics practice artificial birth control? Why do so many Catholics flout their church’s teaching on abortion?
For that matter, if Catholicism is true, why have traditionally Catholic countries in Europe become so secularized?
Also, at the risk of stating the obvious, truth may have little to do with what people believe. For one thing, people are born into communities. Over a lifetime, they frequently relocate from one community to another, viz. from the nuclear family to high school to college, &c.
Because human existence is a communal existence, we have a natural tendency to assimilate to community standards, be it the family in which we were raised, the schools we attended, the business we work at, the town we live in, &c.