Because He was God. As God, He did not have a sin nature.
Let each of you talmidim look after not only your own interests, but also the best interests of others [2:21; 1C 10:24,33].
5 Let this mind be in you which was also in Rebbe, Melech HaMoshiach Yehoshua,
6 Who, though existing in the demut of the mode of being of Elohim [His etzem or essential nature, Yn 1:1-2;17:5], nevertheless Moshiach did not regard being equal with G-d as a thing to be seized [BERESHIS 3:5],
7 But poured out and emptied himself [2C 8:9], taking the demut of the mode of being of an eved [YESHAYAH 52:13- 53:12 [T.N. see the AVDI TZADDIK TZEMACH DOVID MOSHIACH YIRMEYAH 23:5;ZECHARIAH 3:8], and was born in the likeness of Bnei Adam [Yn 1:14;Ro 8:3; MJ 2:14-17], and having been found in appearance as an Adam,
8 Rebbe, Melech HaMoshiach humbled himself and took the path of shiflut (lowliness), unto mishma’at (obedience 2:12) [cf.BERESHIS 3:17] even unto death [Yn 10:17; MJ 5:8; 12:2], and that, a death on HaEtz [the Tree, DEVARIM 21:23; 27:26; Ga 3:13; Pp 3:18].
9 Therefore, also Hashem exalted [YESHAYAH 52:13; 53:12; DANIEL 9:26; 7:14; Ac 2:33; MJ 1:3] Rebbe, Melech HaMoshiach, and gave to him haShem [Ep 1:21; MJ 1:4] above every name,
10 That at haShem of Yehoshua, KOL BERECH (every kneeYESHAYAH 45:23) will bow, of beings b’Shomayim and ba’Aretz and mitachat laAretz (in the world below),
11 And KOL LASHON (every tongueYESHAYAH 45:23) shall make hoda’ah (confession) with an Ani Ma’amin that is an open and public admission that Rebbe, Melech HaMoshiach Yehoshua (Yeshua) is Adoneinu, to the kavod of Elohim Avinu.