Why does God have no trust to his angels?
The Book of Job does not provide a portrait of God's mind that is comprehensive or detailed enough to allow identifying God's specific reasons(s) for putting no trust in his angels.
What one can answer, though, is whether God's distrust in his angels is epistemologically and factually supported (as per the narrative in chapters 1 & 2). And the answer is in the affirmative, notwithstanding that God's distrust in his angel(s) is notoriously inconsistent: In line with one of the comments, God actually trusted Satan at least twice to unjustifiably harm Job under pretext of the wager between God and Satan.
God's concept of Job was that of "a perfect and an upright man" (Job 1:8, KJV). Satan was given the opportunity to prove that Job's devotion to God is conditional on profit. Satan's proposition turned out to be wrong, for "[i]n all this Job sinned not, nor charged God foolishly" (1:22) and "[i]n all this did not Job sin with his lips" (2:10). Thus, it was epystemologically correct for God to distrust his angel.
(Apropos of --and contrary to-- the other answer, this epystemological accuracy validates Eliphaz's declarations in 4:18 and 15:15, whence one cannot categorically state that Eliphaz's views "are flawed").
Why are the heavens not pure in God's sight?
Without foreclosing alternative explanations beforehand, the Tanakh might shed light on the latter half of 15:15:
וְ֝שָׁמַ֗יִם לֹא־זַכּ֥וּ בְעֵינָֽיו
The וּ appended to זַכּ֥ suggests allusion to His purity (זך: pure), whence I posit that the clause could be paraphrased as: And in His eyes, the heavens are not [at the level of] His purity.
Thus, rather than labeling the heavens as impure in themselves, the intended meaning of 15:15 might be to expresses God's superiority over any thing (or something) He has created: in this case, the heavens.