Why elemental nitrogen exists as diatomic molecule

i think its due to the absence of d orbital in nitrogen.

Are you sure ?

i think N₂ molecule contains a triple bond i.e. N≡N ,which is very stable due to effective p\pi - p\pi overlapping and the bond dissisiation energy is also too high for this conformation ; thats why also N₂ is inert at room temperature.

if it'll form higher structures , then it'll form non-planar bonds ,being much reactive at even room temperature.thus it has tendency to form dis molecule only.