Why was the fact that Catholic Bibles were written exclusively in Latin a cause of the Protestant Reformation?
Writing the Bible in a secret language that only priests could understand gave clergy license to interpret scripture in a way that favored giving the Church more power.
Within the hierarchy of the Roman Catholic Church is the assumption that the Church is the sole rightful interpreter of scripture, and that the laity should just stop asking questions and do what they're told.
Human nature being what it is, the Church fathers tended to interpret scriptures in ways that gave more property, power and money to the Church. Any parishioners who found their interpretations to be dubious would have to overcome some barriers, like learning Latin and studying theology.
This led to some corrupt practices, some of which Martin Luther detailed in his 95 Theses.
The Catholic Church is unique among Christian denominations in that it doesn't particularly encourage its adherents to read the Bible . I went to Catholic schools in the 1960s, shortly after the reforms of Vatican II , and used the Daily Missal (an annotated Bible for daily mass) that my older brother had used some years before. The other kids' missals were in English but mine was in Latin. Using the vernacular in mass was that recent a phenomenon in the culture.
Many of the doctrines of the Catholic Church were based on the Latin Bible containing the Deutro Cannon. The availability of Greek Bibles contradicted the teachings of the Catholic Church.
One of the main causes of the Protestant Reformation was the teaching of the Catholic Church regarding the sale of indulgences.
The Latin Bible was used to support the teaching of the sale of indulgences. The Greek Bible which had become available in Europe before the Protestant Reformation, did not support the teachings of the Catholic Church about the sale of indulgences and other doctrines.
The translations of the Bible in languages that people could read for themselves, led to the Protestant Reformation.