Because it was a top secret, perfectly hidden like the death and rape camps were. Lack of evidence is a proof of Serbian perfidy. I am sure Hoplite, Fusion or A-brit can explain it to you.
None of the three were secret, nor too terribly well hidden.
I expect to read the Serbian Academy of Sciences and Arts Memorandum and hope to read the RAM plan when it gets translated.
As to Milosevic and his culpability, I expect to hear more about intercepted phone calls in the days to come - we don't pour billions into the NSA to not hear other's conversations.
Lions and Tigers and Echelon, oh my!