Question

ionic compounds are more soluble in water than benzene. why

Answer 1

Water has a high dielectric constant. (It is able to break the ionic bonds between the ionic compounds. Eg: It separates the $Na^+$ and $Cl^-$ ions of NaCl.) Hence it is a polar covalet compound and able to dissolve ionic compounds easily.
Whereas Benzene lacks the proper dielectric constant as water and hence can only dissolve a few substances (Such as Rubber).
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Answer 2

As water is a polar covalent compound it decreases the electrostatic forces of attraction , resulting the free ions in the aqueous solution