This is because even-numbered alkanes pack well in the solid phase, forming a well-organized structure which requires more energy to break apart.
ðɪs ɪz bɪkˈʌz ˈiːvənnˈʌmbɚd ˈælkeɪnz pˈæk wˈɛl ɪnðə sˈɑːlɪd fˈeɪz, fˈɔːɹmɪŋ ɐ wˈɛlˈɔːɹɡənˌaɪzd stɹˈʌktʃɚ wˌɪtʃ ɹᵻkwˈaɪɚz mˈoːɹ ˈɛnɚdʒi tə bɹˈeɪk ɐpˈɑːɹt.