Common answer: Because the Catholic Church wanted to keep control over all the people and knew that once lay Christians could read the Bible for themselves they would see all the false teachings the Church had added onto the Bible. Fortunately, Martin Luther broke the Church of Rome’s power by translating the Bible into German so that lay Christians could liberate themselves by rediscovering the Biblical truths over Rome’s lies.
I think especially amongst Protestants that both the implicit assumption in the question itself and the answer are “well-known”, but I want to challenge these ideas on two fronts:
1. Firstly, Luther’s translation of the Bible into German was not the first one; there were many Catholic translations well before him. Further, St. Jerome translated the Bible into the Latin Vulgate in the first place 1,000 years prior to Luther as Latin was the dominant tongue.
2. Secondly, and to me a bit more interestingly because I recently read the history of these two great saints’ lives, Sts. Cyril and Methodius had made translations of the liturgy and the Bible for the Slavic peoples back in the 800s!
Dr. Warren Carroll’s The Building of Christendom covers their lives and their missionary journeys and talks about this fact. Providentially, Pope Benedict just one month ago dedicated his Wednesday audience to these two saints’s lives:
On the way they [Sts. Cyril and Methodius] stopped in Venice, where they had a heated discussion with the champions of the so-called “trilingual heresy” who claimed that there were only three languages in which it was lawful to praise God: Hebrew, Greek and Latin. The two brothers obviously forcefully opposed this claim. In Rome Cyril and Methodius were received by Pope Adrian II who led a procession to meet them in order to give a dignified welcome to St Clement’s relics. The Pope had also realized the great importance of their exceptional mission….Thus he did not hesitate to approve the mission of the two brothers in Great Moravia, accepting and approving the use of the Slavonic language in the liturgy. The Slavonic Books were laid on the altar of St Mary of Phatmé (St Mary Major) and the liturgy in the Slavonic tongue was celebrated in the Basilicas of St Peter, St Andrew and St Paul.
Isn’t that heresy interesting? If the Catholic Church were intent on controlling the people via languages used, the trilingual heresy would have served her well, preventing foreigners who became Christians from being able to read the Bible and talk to God–it would all have to be “mediated” through the Church’s priests, but as the Church does not have this diabolical design, she rightly condemned this teaching for what it was–heresy.
The saints wanted the Slavic people to understand divine revelation so much that they, in the words of Benedict:
Desirous of imitating [St.] Gregory [Nazianzen] in this service, Cyril asked Christ to deign to speak in Slavonic through him. He introduced his work of translation with the solemn invocation: “Listen, O all of you Slav Peoples, listen to the word that comes from God, the word that nourishes souls, the word that leads to the knowledge of God”….The need for new graphic characters closer to the language spoken was therefore clearly apparent: so it was that the Glagolitic alphabet came into being. Subsequently modified, it was later designated by the name “Cyrillic”, in honour of the man who inspired it. It was a crucial event for the development of the Slav civilization in general. Cyril and Methodius were convinced that the individual peoples could not claim to have received the Revelation fully unless they had heard it in their own language and read it in the characters proper to their own alphabet. (emphasis mine)
They actually greatly improved the Slavic language so that they could better grasp these divine truths!
Jumping back to Luther and the Reformers, the well-known answer to this (what I have shown to be false) question is also wrong in how it describes Luther and the other Reformers’ views on whether lay Christians should read and interpret Scripture for themselves. Protestant historian Alister McGrath bursts the bubble:
The magisterial Reformation initially seems to have allowed that every individual had the right to interpret Scripture; but subsequently it became anxious concerning the social and political consequences of this idea. The Peasant’s Revolt of 1525 appears to have convinced some, such as Luther, that individual believers (especially German peasants) were simply not capable of interpreting Scripture. It is one of the ironies of the Lutheran Reformation that a movement which laid such stress upon the importance of Scripture should subsequently deny its less educated members direct access to that same Scripture, for fear that they might misinterpret it (in other words, reach a different interpretation from that of the magisterial reformers). For example, the school regulations of the duchy of Württemberg laid down that only the most able schoolchildren were to be allowed to study the New Testament in their final years – and even then, only if they studied in Greek or Latin. The remainder – presumably the vast bulk – were required to read Luther’s Lesser Catechism instead. The direct interpretation of Scripture was thus effectively reserved for a small, privileged group of people. To put it crudely, it became a question of whether you looked to the pope, to Luther or to Calvin as an interpreter of Scripture. The principle of the ‘clarity of Scripture’ appears to have been quietly marginalized, in the light of the use made of the Bible by the more radical elements of the Reformation. Similarly, the idea that everyone had the right and the ability to interpret Scripture faithfully became the sole possession of the radicals. (emphasis mine)
(The above reference is from Alister McGrath‘s Reformation Thought, which I saw referenced from a Called to Communion comment. Also, the “radicals” McGrath speaks of are the radical reformers, for example the Anabaptists.)
So, the fact is that the Catholic Church did not prevent vernacular translations and instead there were ones for 700 years before the Reformation.