good question! my ans is NH3 is stable structure(possible structure) and N2H6 is not. in your proposed N2H6, both the nitrogens in NH3 needs to be bound together with a covalent bond. if so, nitrogen gets four covalent bonds which is a violation of valency bond theory. and in addition, as nitrogen contains a lone pair of electrons, they cannot try to make a bond with another nitrogen because of the repulsions between the electrons of two N atoms. so these restrictions restricts your proposed N2H6 and so doesn,t exist.