Why do alkenes and alkanes have different boiling points?
Because alkenes and alkanes have different forces of intermolecular attraction.
Of course, there are many different alkanes, and many different alkenes. In general, their boiling point depends on chain length, with the longer chain-lengths giving rise to HIGHER boiling points. Why? Because the molecules can interact along the chain, and the longer the chain, the greater the degree of interaction, and the greater the boiling point.
We can appreciate this by interrogating the boiling points of the alkane series: