Why do alkenes have geometric isomers and while alkanes and alkynes do not?
Sigma bonds can rotate around and shift. This makes isomers of non-substituted alkanes impossible because the atoms can shift back and forth.
Pi bonds, like those in double bonds, can't freely move around. Since each carbon in a double-bond chain can form another bond that doesn't move, you can create symmetrical isomers.
Alkynes are linear and can't have a substituted atom branching off. Carbon can form no more than four bonds.
Edit: Alkanes can have isomers if one of the carbons has both of its hydrogens replaced with different substituents.