If the Geneva Bible was the Bible of the English reformation, then why did the KJV become so widely used? If the Geneva Bible was the bible of the reformation, Why is the KJV used more widely than the Geneva Bible? And (if applicable) what would be the main differences between the two bibles (in terms of biblical content rather than the preferences of a presiding king)?
Wikipedia gives a summary of the history.

The Geneva Bible translation was produced about 50 years before the King James, and was used by most English-speaking Protestants for much of the Reformation. However the Geneva Bible incorporated "annotations which are an important part of the Geneva Bible [and] were Calvinist and Puritan in character, and as such they were disliked by the ruling pro-government Anglicans of the Church of England, as well as King James I". 

King James therefore commissioned an alternative translation (which also did not include the notes). The KJV was adopted by the Church of England, which constituted by far the majority of English-speaking Protestants. This adoption caused the translation to be by far the most widely available English translation over the next 300 years, and accounts for its enduring popularity.