Why has it been argued that Chaucer is a turning point in Medieval literature?
Three central circumstances mark Geoffrey Chaucer as a turning point in Medieval literature. (1) The other two prominent poets of the mid-to-late-1300s were either wholly or virtually anonymous, being the unknown poet of Sir Gawain and the essentially unknown Pearl Poet of Piers Plowman (hinted at as being Will Langland), while Chaucer was very well known and in the employ of, then later the appointed poet of, the courts of three English kings: Edward III, Richard II and Bolingbroke called Henry IV. Thus even the lifting of the veil of anonymity marked a turning point.
(2) Chaucer was the first poet of renown and reputation to write multiple works of poetry in the Middle English vernacular using the London dialect as his poetic language. The Pearl Poet (Piers Plowman) had also written in Middle English, in a scribal dialect of Worcestershire, yet his work is limited and he is essentially anonymous. The Sir Gawain poet wrote in English vernacular but in a dialect very different from both Pearl's and Chaucer's. These three excerpts show a comparison between the dialects used for Piers Plowman, Chaucer's Troilus and Criseyde, and Sir Gawain. The first two are actually readable to some extent while the third is not.
In a somer seson, whan softe was the sonne,
I shoop me into shroudes as I a sheep were,
In habite as an heremite unholy of werkes,
Wente wide in this world wondres to here. (Piers Plowman)
* * * *
The double sorwe of Troilus to tellen,
That was the king Priamus sone of Troye,
In lovinge, how his aventures fellen
Fro wo to wele, and after out of Ioye,
My purpos is, er that I parte fro ye. (Chaucer, Troilus and Criseyde)
* * * *
Siþen þe sege & þe assaut wat3 sesed at Troye,
Þe bor3 brittened & brent to bronde3 & aske3,
Þe tulk þat þe trammes of tresoun þer wro3t,
Wat3 tried for his tricherie, þe trewest on erthe; (Sir Gawain)
(3) Chaucer is the first English poet to travel to Italy and France and return with great poetry from those countries. He brought The Romance of the Rose (Roman de la Rose) from France. He brought the tale, written by Boccaccio, of Troilus and Criseyde from Italy (c. 1373 while on a royal mission). He brought back influences of both Dante and Petrarch from Italy. This is a third dominant reason that Chaucer marks a turning point in Medieval English poetry. When these particulars are added to his body of work, from The Book of the Duchess to The Canterbury Tales and all else in between, including "The Complaint of Chaucer to his Empty Purse," written to Henry IV, it is clear there is significant reason to mark a change in Medieval literature from the works of Chaucer.
Troilus and Criseyde