Why Chaucer is called the Father of English poetry?

Chaucer is given this title for a few reasons.  First, he
is one of the first English poets that we know by name.  During this time period, it was
common not to know the author's name and label it as "anonymous," largely due to the
tradition of passing stories along orally through time from one generation to the
next.

Second, we give him the title because of his
accomplishments.  His largest and best known is The Canterbury Tales.  In this work
(which is not finished as he died before its completion), Chaucer creates a frame story
with his poetry.  The outer story is the pilgrimmage that the group is taking to pay
homage to Thomas Becket at Canterbury.  The inside story is a collection of short
stories (the first of its kind which morphs into the short story as we know it today)
focusing on the individual travelers.

Third, Chaucer,
through his own life experiences, is able to give us a picture of life in his age for
all levels of society.  His Tales give us the knight (highest ranking of the travelers)
to the plowman (revered in Chaucer's time for their importance in providing food for the
public, but certainly not considered as a wealthy or sophisticated member of society). 
Not many authors have ever been able to do this well, and it was several hundred years
before Shakespeare came along and was also able to successfully represent all members of
society in a straightforward and truthful way while still being
entertaining.