When does a solute form ions?
In water, HCl reacts to form $\ce{H3O+}$ and Cl-.  

Then, water molecules surround the respective ions in a stabilizing orientation.
The partially negatively charged O atoms of water interact with the $\ce{H3O+}$.
The partially positively charged H atoms of water interact with the Cl-.

None of the above can occur in benzene because benzene is very reluctant to accept a proton and because benzene is non-polar.

Generally speaking, polar solvents have an ability to stabilize ions that nonpolar solvents do not.