sp2 hybridized carbon versus sp3 hybridized silicon Carbon's most commonly found allotrope is graphite/soot/fullerene/carbon nanotubes which are all $\mathrm{sp^2}$ hybridized forms of it, while silicon being its first periodic twin is mostly found in the $\mathrm{sp^3}$ hybridized form in bulk, while formation of silicene is extremely difficult. While in the case of carbon, diamond is the one which is harder to find, what is the reason for this discrepancy?
Carbon — and all the other elements of the second period, for that matter — is exceptionally good at forming double bonds. A double bond is formed if there is a &#x3c3; bond and a &#x3c0; bond. Forming a &#x3c3; bond is not hard: You have two orbitals coming from two different atoms pointing at each other. Forming a &#x3c0; bond is much harder, since the orbitals are pointing orthogonally away from the bond-axis. Thus, the overlap of &#x3c0; bonds is often a lot weaker than for &#x3c3; bonds.

This gets more important if you consider third-period elements rather than second-period ones. They have a total of eight more core electrons making them much larger in size and the orbitals more diffuse. A &#x3c3; bond is still formed easily, there will be the point when two diffuse orbitals are close enough to interact in a &#x3c3; fashion. To arrive at favourable interactions for a &#x3c0; bond would require the atoms to come much closer than what would be comfortable; unfavourable interactions between the core electrons would already be a thing.

In fact, I was taught in my Concepts of Inorganic Chemistry class, that disilene $\ce{Si2H4}$, the heavier homologue of ethene, does not form a classical &#x3c0; bond. Instead, it can be understood as a par of two singlett silene fragments $\ce{SiH2}$ which approach each other so that the empty p-orbital of one silene can interact with the lone pair in an $\mathrm{sp^2}$ orbital of the other silene fragment. Obviously, the symmetry is lower and the bond is much weaker.

So the reason behind it is that $\ce{Si=Si}$ double bonds are hard to create and this gets even harder for heavier homologues while $\ce{C=C}$ double bonds are reasonably stable and easy to generate.