Why is Chaucer called the "father of English poetry," in detail?

There are several reasons why Chaucer is called the "father of English poetry." The first is that histories of literature for many centuries had preferred, primarily for pedagogical reasons, to create small canons of monumental poets rather than trying to teach a wider range of poets. This was in part designed to create a shared cultural canon. More recent critics would argue that there is no one "father" of English poetry but a cluster of poets who were important during the period of the formation of middle English literature. Older works of criticism, though, still cleave to the notion of the individual genius (usually a white male) who serves as a "father" to a literary movement.
Chaucer was especially important in his contributions to the evolution of iambic pentameter as the predominant meter of English verse. Metrically, this meant synthesizing the Anglo-Saxon strong stress tradition, a form in which lines consisted of two accented syllables, a caesura, and two more accented syllables, with any number of unaccented syllables and a fixed alliterative pattern, with the syllabic patterns of French verse. This led to the accentual-syllabic prosody still used today.
He was also important for the sheer variety and volume of his work and for being one of the poets instrumental in the rise and refinement of the vernacular as a medium for poetry.