Why are pentahalides more covalent than trihalides ?

The elements of group 15 have five electrons (two in the s-and three in the p-orbitals) in their respective valence shells. Since it is difficult to lose all the three electrons to form E³⁺ or even more difficult to lone all the five valence electrons (two s-and three p-) to form E⁵⁺ ions, therefore, higher elements have no tendency to form ionic compounds. Instead they form covalent compounds by sharing of electrons. 

Since elements in the +5 oxidation state have less tendency to lose electrons than in the +3 oxidation state, therefore, elements in the +5 oxidation state have more tendency to share electrons than in the +3 oxidation state. Thus, elements in the +5 oxidation state are more covalent than in the +3 oxidation state. 

In other words, pentahalides are more covalent than trihalides.