As noted in class, the pilgrims are roughly divided into the three acknowledged categories of feudal society - the three "estates" - with the church at the top, followed by nobility, and finally peasantry. By Chaucer's time period, with the breakdown of the feudal order, this third classification was broadening to include the new merchant (or "middle") class.
Fittingly, many of the pilgrims do not fit neatly into any of the three categories, and the two women reveal their inability to conform to their expected roles, as well. Furthermore, Chaucer saw corruption within the church, especially, so he used the Tales as a way to satirize some of the more "religious" pilgrims. As Chaucer himself benefited from the patronage of the nobility or upper class, it follows that his portraits of the knight and gentry are more positive in tone.
In short, the Prologue introduces us to medieval society at a time when it was rapidly changing - due to religion, politics, disease and technology - and it still serves as perhaps our best and most comprehensive lens into that world.
Source: The Three Estates