N2H4 can't have a double bound because this would lead to an unequal distribution of charge and product an overall compound containing a type of charge state rather then a neutral compound.
Must consider what you learned in general chemistry in regards to Lewis structure, bonding and the octet rule. In terms of hydrogen, hydrogen contains one valance electron that it is willing to donate to form a chemical bond (which it is why its in group 1 and thus has a 1s orbital). Whereas Nitrogen contain five valance electrons and wants to form three bonds (contains an electron configuration of 1s^1 2s^2, 3p^3).
when bonding occurs we see we have a nitrogen bonded to another nitrogen through a single bond with each nitrogen respectfully bonded to two hydrogens (this arrangement completes the octet rule for hydrogen and the bonding requirement for nitrogen). Each nitrogen contains a pair of unbounded electrons which cause a repulsion force between both sets of electrons on each nitrogen pushing them as far apart a possible and completing the octet rule of both nitrogen's leading to a neutral molecule.