This is due to the greater size of the P3− and As3− ions, so that less lattice energy needs to be released for the salts to form.
ðɪs ɪz dˈuː tə ðə ɡɹˈeɪɾɚ sˈaɪz ʌvðə pˈiː θɹˈiː mˈaɪnəs ænd æz θɹˈiː mˈaɪnəs ˈaɪɑːnz, sˌoʊ ðæt lˈɛs lˈæɾɪs ˈɛnɚdʒi nˈiːdz təbi ɹᵻlˈiːst fɚðə sˈɔlts tə fˈɔːɹm.