Why does it require so much pressure to create diamonds?
There are two allotropes of carbon crystal, graphite and diamond.   Both
are stable, so the formation of diamond structure is competitive with
the other structure (or structures, if you include buckyballs
and tubes).   The important consideration, therefore, is crystal growth rate,
which will depend on solvent and seed material, as well
as temperature and
pressure.

For manufacture of large artificial diamonds, liquid iron 'solvent' is
used, which is why high temperature is important; carbon has to diffuse through the liquid solvent. At high temperature,
the (relative) instability of diamond means that
a pressurized environment is required: diamond has to expand (diamond is 3.53 g/cc, but graphite is 2.23 g/cc) in order to become graphite, and pressure prevents that.   Pressure alone might
become so high that graphite recrystallizes as denser diamond, in a shock wave (as long as you only want to make small diamonds) made with explosives.  At elevated temperatures and low pressure, diamond
recrystallizes as graphite, however.

It is
possible to grow diamond films (depending on a substrate that
seeds the crystal form of interest) at low pressures as well, from
vapor.   Pyrolyzing CH4 in the presence of a cold surface that doesn't
seed the graphite form is the general approach. 

To summarize: in growth of a carbon crystal, the environment determines
the dominant crystal structure in a number of ways.  The Diamond Makers by Robert Hazen, is a good treatment of this subject.