Why are silicon analogs of alkenes or alkynes so difficult to make?
The typical Si–Si single bond length in a silane is around 2.33 Å.  This is much longer than a typical C–C single bond (~1.53 Å) and helps explain why silicon-silicon single bonds are so much weaker than carbon-carbon single bonds (bond dissociation energy: ~53 kcal/mol for silicon-silicon vs. ~83 kcal/mol for carbon-carbon single bonds).  When we try to form a silicon-silicon double or triple bond, the large separation between the two silicon atoms results in even less effective p-orbital overlap and therefore even weaker π-bonds.  The result is that while disilenes and disilynes are known (see the linked SE Chem posts), they are extremely reactive.  Only when bulky substituents are placed around the double or triple bonds (to provide some "steric protection" against further reaction) can the molecules be isolated and characterized.