Why is Chaucer called the father of English poetry?

Answer:

No one reading this has escaped quotes like:

"Time and tide, wait for none."

or that,

"Love is blind."

All that was Chaucer speak. So, instead of copy pasting Chaucer's biography excerpts, let me concentrate on the 'Why' in your question and answer with the 'Because's'-
Because, in the times when Chaucer walked, it was not fashionable for serious poets to write in English.
Because, Chaucer established rhyming couplets as the main narrative system in English. During Chaucer's lifetime there was a serious attempt to re-establish the old alliterative system as the main narrative method in English poetry. It was mainly thanks to Chaucer that English adopted the rhyming system.
Because, Chaucer was a major force in establishing the dominant meter for English poetry, and in introducing rhyme as the main way of writing a poem.
Because, Chaucer more than any other writer is the reason English literature turned out the way it did.
Also because, by composing in the vernacular – the everyday language spoken in London and the East Midlands – Chaucer lent respectability to a language that would develop into the medium for one of the world’s greatest bodies of the literature.
In this sense he is indeed the father of the English poetry.