Why is diborane difficult to represent with an ordinary Lewis structure?

Diborane ($B_2{H}_6$) is difficult to represent with an ordinary Lewis structure because it is an electron-deficient compound.

To link its eight atoms together, the molecule requires at least seven bonds; however, it possesses only 12 valence electrons, which is sufficient to form only six standard two-center, two-electron covalent bonds. Because there are fewer than two electrons available per bond, the molecule cannot be accurately depicted using the traditional Lewis approach, which relies on the concept of electron pairs localized between two specific atoms.12

Instead, the structure of diborane is characterized by multicentre bonding, specifically three-centre, two-electron bonds. In these bridges, a single pair of electrons is delocalized and shared among three atoms—two boron atoms and one hydrogen atom—rather than being restricted to a single pair of atoms. This delocalization allows the molecule to remain stable despite having fewer electrons than would be required by conventional bonding rules.3452