I'd guess two reasons:
It's not 'proper' for a new administration to trash the prior one or to point out the glaring errors "we" pulled in foreign affairs. (Don't hear much from this or any other administration about Carter's disaster in Iran).
And,
If you ignore the reality of who was who in FRY...it fits into the pattern of heroic US actions to restore or create popular governments where they did not exist before. (Meaning, official pronouncements still link invading Serbia with liberating Iraq)
Besides, it was SOOOO easy!
Because the cat got their tongue or because they do know more than they are willing to admit?
Don't forget Bill Kristoll's slip ("Crush Serb skulls")