Because the printing press was not in use in that period
Answer: In Short, Books were handwritten and very expensive.
they were written on parchment and made of animals skins.
Explanation :You don't need to know this and its probably not in your lesson but its good to know. There are many reasons for this. One being the church. The church regulated what the general public learned and made it so people couldn't rise up in the ranks and become educated. So only the nobles read books. Independent study was discouraged and the only people who wrote books were mostly people of the church.
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