Question

why is sigma bond is more stronger than pi bond

Answer 1

Pi bonds are usually weaker than sigma bonds. Quantum mechanics says this is because the orbital paths are parallel so there is much less overlap between the p-orbitals. Pi bonds happen when two atomic orbitals are in contact through two areas of overlap.

Answer 2

As sigma bonds permit for the electronic structure to be shared to a much greater degree between both the two nuclei, Pi bonding is normally poorer over sigma bonds.

Explanation:

Quantum theory claims it's because the angular directions are opposite, so the p-orbitals intersect even less. If two charged particles are in communication across two regions of overlap, Pi bonds exist.