Question

why is Diamond rarer in nature than graphite?

Answer 1

Diamond is considered to be hardest element present on earth.
diamond take million of years to be formed.

Answer 2

The tetrahedral bonds in diamond are much stronger and more stable than the planar bonds in graphite.  But it takes more energy to push the carbon atoms into the tetrahedral structure than into the planar structure.  Graphite forms first as heat or fire removes hydrogen, oxygen and other atoms from a 'pool' of hydrocarbon compounds.  Graphite can be formed from simple burning.  Soot is graphite, although not super pure.  Then, where there's lots of heat and pressure, some graphite is transformed into diamond.  

So diamond is most fundamentally more rare because it takes more energy to form the structure.  And it's also rarer because it's formed from graphite by a process that only transforms a small fraction of graphite into diamond.

To make these qualitative arguments quantitative, you need to calculate the electronic energy bands and total energies of the different structures.

It takes a lot more work to produce a diamond than to produce graphite.  Diamonds are produced in nature way underground where the pressures and temperatures are very high.  

Here is how natural graphite is formed:  (from the Wiki article)

There are three principal types of natural graphite, each occurring in different types of ore deposit:

Crystalline flake graphite (or flake graphite for short) occurs as isolated, flat, plate-like particles with hexagonal edges if unbroken and when broken the edges can be irregular or angular;

Amorphous graphite occurs as fine particles and is the result of thermal metamorphism of coal, the last stage of coalification, and is sometimes called meta-anthracite. Very fine flake graphite is sometimes called amorphous in the trade;

Lump graphite (also called vein graphite) occurs in fissure veins or fractures and appears as massive platy intergrowths of fibrous or acicular crystalline aggregates, and is probably hydrothermal in origin.

None of these processes involve the kind of energy that the formation of diamonds require.