Answer:
The Canterbury Tales was a popular book of the period because it was written in easy-to-understand language. This was because it was written in Vernacular English language, at a time where most tales were written in Latin, French or Italian. Chaucer at his time was part of the time of this trend where English-language stories were taking off. So basically, the languaged he use was trending, and he hit a million-subscribers extremely fast.
