Why are some substances not able to dissolve in water?

Water is a polar molecule. If a solute dissolved in water is polar molecule, it will dissolve in water. If a solute dissolved in water is non-polar like oil it will not dissolve in water. Polar dissolves in polar.

Solubility is how you measure the ability for something to dissolve. However, somethings are insoluble in water because of their polarity.

Polarity plays a large amount in determing solubility of a certain substance.

There are also factors that can change something from being barely soluble to very soluble (which is why you wash you hands with hot water).

Its due to the bonding of water and intermolecular forces.

Water has a hydrogen bond which is strong enough to hold together the water particles, but weak enough that it can be broken apart.

This is why grease which is nonpolar can not be dissolved in water and why we use soap when washing our hands (especially with grease or other solutes that wont dissolve in water)