The way I understand it is that they were originated because of a lack
of skilled workers - up until then boats had been quite "shapely".
These box like vessels were easier to build. On the water however they
lacked performance. In part due to the barge racing and combined with
the need to travel out of the rivers we can then see the "snib" bow
deleoped. This development was not the round bow that we know today and
usually they still had the "budget" stern. This then gave them better
handling charecteristics in open water.
I'm afraid this does not really answer your question! I would assume
that they were pigs in open water not only because of there shape but
the sails they carried. The rig and the craft developed hand in hand
with the need to travel further but still get into those shallow
waters. The more one delves into the history of these craft the more
questions seem to be left to answer.