After the Norman Conquest of 1066 in which William the Conqueror of Normandy (part of France) and his Norman noblemen eradicated Harold of Hastings and the Saxon nobles, French became the official language in England. All written works were put in French including literature. (For instance, the Arthurian Legends are in French).
Then, Geoffrey Chaucer, whose last name is the French form of the word shoemaker, came along in the second half of the thirteenth century (1343-1400). Chaucer, who was bilingual, as were many in his time, was greatly influenced with the matter and style of French poetry. However, when he realized how people of his time delighted in stories, he gave these to them. Thus, the Canterbury Tales began. Perhaps because this work is about the pilgrims whom Chaucer had observed so often in England, he put the tale in their language of usage. Also, there was a bias toward French and an English work could be easily more popular.
According to luminarian.org, Chaucer's influence on the English language is overrated as it was a natural progression for the written language to become English. Nevertheless, the tremendous popularity of "Canterbury Tales" certainly contributed to this progression. His assigning of full value to every syllable in the lines in Canterbury Tales had to have had an effect upon the development of Modern English. Certainly, too, the vocabulary and meanings of words that have originated from French are present in his works; many of these words remain in Modern English (60% of the words in Modern English originate from French).
Chaucer is also known for metric innovation. It is he who initiated the use of iambic pentameter into popular works. He also employed rhyming couplets. His poetry is credited with helping to standardize the London dialect of Middle English, although some linguistics think that the influence of the court was a more powerful influence on the changes made. The Oxford English Dictionary credits Chaucer, with his ear for common speech, as employing many current English words into his works, thus furthering the development of Modern English by way of Middle English. Of course, the fact that "The Canterbury Tales" was one of the first books to be published and widely read has contributed to the powerful influence of Chaucer upon the English language.