Why is Geoffrey Chaucer called the 'Father of Modern English Poetry'?

Apart from the fact that The Canterbury Tales and Troilus and Criseyde are outstanding works artistically, there are several reasons Geoffrey Chaucer is considered the father of English literature.
Over a period of 300 years, from the Norman Invasion to Chaucer's time, the English language had undergone a seismic transformation due to the huge number of loan words incorporated into it either from French or directly from Latin and Greek. Despite differences in spelling, pronunciation, and vocabulary, in Chaucer we see the language in a form surprisingly close to our own. Many fine translations have been made of his works into modern English, but it's still possible for us to read Chaucer in the original, with the help of footnotes and a glossary, and to recognize that this is essentially our own language. Even when The Canterbury Tales are read aloud with the correct Middle English vowel sounds and the pronunciation of the final 'e' now silent, the words still are obviously recognizable as English and can be largely understood. In fact, one can even say that it is especially when Chaucer's verse is read aloud that it sounds so natural to us, because it's written basically in a conversational tone. The Canterbury pilgrims are talking to each other, much as people on a trip or a tour would do today.
Another basic reason Chaucer is the fountainhead of our literature is that the London dialect in which he wrote became the standard form of the English language. In all European countries, different dialects of the common language were and are spoken. In Italy, the Tuscan language, the dialect of Florence, became standard Italian because of the greatness and prestige of Dante's writings. In England, Chaucer played approximately the same role. No previous author in England had written works of such high artistic value as those of Chaucer, and, unsurprisingly, the speech of Londoners became the literary standard.