Why elemental carbon is solid?
Carbon forms large molecules such as the ring structure of graphite, or tetrahedrons of diamond. Boron, like carbon, forms relatively large covalently-bonded molecules.

Nitrogen, oxygen and fluorine are diatomic, so molecules are less massive. The inert gases He and Ne are monatomic and therefore have lower b.p. yet.

In a similar fashion, metallic bonds hold together Li, Be and other metals.