why alkynes are less polar than alkanes

Actually, your question seems to have a small misunderstanding. Neither alkanes nor alkynes are polar. Both of these types of hydrocarbons are nonpolar because they consist only of carbon and hydrogen atoms bonded together with nonpolar covalent bonds.

Polarity in a molecule arises due to the difference in electronegativity between the atoms forming a bond. In the case of carbon and hydrogen, the difference in electronegativity is very small, leading to a nonpolar bond.

Since alkanes and alkynes are made up of a series of these nonpolar bonds, the entire molecule is also nonpolar. Therefore, it's not accurate to say that alkynes are less polar than alkanes, because both are nonpolar.

However, if you were referring to alkenes and alkynes, it's worth noting that alkenes can be slightly polar if they are substituted with a polar functional group, while alkynes are still nonpolar due to their linear geometry.