Personal Observations
It was ultimately the flat-soled
shoe of that allowed for the 
development
of dancing en pointe. The reasons for this may not seem obvious at first.
However, as one who has done pointe work, I believe I can use my experience
to explain
why pointe work would have been physically more difficult in heeled shoes.
When doing pointe work, a dancer rarely rises to full pointe slowly but
tends to use a quick relevé, which is essentially a discreet hop,
allowing the foot to go from completely flat to fully vertical.
It is in this subtle motion, from horizontal to vertical, that the charm
of pointe work is located.