Here's why:

* Hybridization: In hydrazine, each nitrogen atom is sp³ hybridized. This results in a tetrahedral geometry around each nitrogen, with the lone pairs of electrons occupying two of the four sp³ orbitals. This arrangement leads to a stronger N-N bond due to greater overlap of the hybridized orbitals.

* Electronegativity: Both nitrogen atoms in hydrazine have a lone pair of electrons, which contributes to increased electron density and strengthens the N-N bond.

* Resonance: Diimide (HNNH) can exhibit resonance, which delocalizes the electrons and weakens the N-N bond. The resonance structures contribute to a double bond character in diimide, but the single bond character of the N-N bond in hydrazine makes it stronger.

In summary: The greater electron density, stronger orbital overlap, and lack of resonance in hydrazine contribute to its stronger N-N bond compared to diimide.