Question

Why does the theoretical molecule cyclohexyne have a lot of ring strain?

Answer 1

C-C bonds are ideally at at 109.5°. Cycloalkane rings with fewer than 5 carbons are forced to have smaller C-C bond angles, and so possess significant ring strain and so are unstable.

CΞC bonds are ideally at 180°. Trying to force this to be a smaller angle results in strain. Once you get to 8-membered rings and higher, there's enough inherent "floppiness" brought about by C-C bond rotations that the triple bond can be accommodated.
CΞC has sp hybridysation ie linear structure, but in cyclohexyne it cant be formed as it has less then 8 carbon atom and has lots of ring strain.