Well, the reason today's histories give it so much importance is easy: Luther; we listen to Luther. Protestant partisanship has dominated historiography on late medieval religion and the Reformation so completely that even scholars didn't realize it until the late 20th century. So we unite "sola scriptura" with Luther's German Bible (and other vernacular ones that followed it).

But Luther, like quite a few clerics of his day, had a tendency to substitute propaganda for fact. By 1500, there were eighteen printed editions of the Bible in German. With 300 to 1000 copies per edition, that probably means about 9000 printed vernacular Bibles in High German-speaking areas alone. To say nothing of manuscript copies, individual Testaments and books, plenaries that contained the biblical text rearranged to match what passages were read in church each day...and so on.

True, even vernacular literacy rates were rock-bottom low. (30-50% in the most cosmopolitan of cities; negligible in rural areas). But in the 15th century, sermons were massively popular public events. And at least according to surviving books of sample sermons, priests--especially if they didn't have time--were encouraged to explain the Latin reading in German...or, if they didn't have time or didn't have the education, to just read the darn thing. In German. 15th century priests absolutely placed a premium on people being able to understand lessons. (More so, it seems, than in earlier centuries). And the literate population? Some of the most literate groups included nuns, monks, and priests--many of whom fought bitterly to stay subject to Rome.

Oh, right, and the years around 1500 are widely accepted in scholarship as the most ecclesiastically devout in the Middle Ages--the most loyal to the Church.

Right.

...Except...then...when Miriam Chrisman looked at middle-class lay-written propaganda from the early Reformation, she found that the ONE uniting feature in all of them was the need for the "word of God." When rural peasants revolted against the nobility in 1525, they appealed to "God's law" and "God's word" as the legal justification for their rebellion.

Well, crap.

So, scholars are just beginning to study the impact of the vernacular Bible in the fifteenth century, and there isn't really any major work on the jump from that to the Luther movement. I think you can say, however, that it wasn't so much the Bible in the vernacular that caused the Reformation, or revolutionized the church, or however one might phrase it. Instead, it was more the idea of the Bible as something (a) accessible, and (b) sola--that is, the only authority.

"Das wort gottes", in a sense, gave people immediate access to God, salvation, and even power on Earth--or at least, gave them that hope.

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Further Reading: Anthony Gow is probably the best scholar on the 15th century Bible in High German (Luther's language). Miriam Chrisman's Conflicting Visions of Reform is an excellent English-language overview of lay propaganda pamphlets from Germany.