Dative bond between oxygen atom from water and mercury atom is short. Ionic contact between nitrate oxygen atom and mercury atom is longer. I drew only the shortest contacts from crystal structure of mercury(II) nitrate dihydrate, but note this is not one molecule. There are two nitrate anions and one Hg2(H2O)22+ cation.
Concerning primary question - scientist cannot simply explain why mercury(I) form metal-metal bond.
So this is called intristic property of this element. Other elements (but probably not all) also can form chains or rings consisted of bonds between atoms of the same element (eg. B, C, N, O, Si, S, Se and so on)