Why is ammonium nitrate so water-soluble?
Do you ask why ammonium nitrate has a high water solubility......?
You should check the literature (or the webz) on the solubility of ammonium nitrate in water. It is VERY large, and as a physical scientist you should always interrogate data. Ammonium nitrate is a substance with a large enthalpy of hydration, but a fairly low lattice enthalpy; certainly its lattice enthalpy compared to say sodium chloride is low.
So why should the lattice enthalpy be low? Both ammonium,