Question

why diamond and graphite has a high melting point

Answer 1

Because the intermolecular space between the atoms of these elements are strongly packed and are very close to each other.

Answer 2

In a diamond every carbon atom connects with four more carbon atom with single sigma
bounds .In graphite, every carbon atom bounds with 3 other carbon atoms and gives it fourth electron into the delocalised electron field.Both of them have crystal sructre.