[Pontiac]

The post-mortem would certainly have brought forth conclusive evidence to bring charges and a conviction.

If the coroner did not mess up the evidence. Or the child did not wash it away. Or the evidence did not support the DA’s case.

These things and others could have made it advantageous not to bring up physical evidence.

[GovernmentShrinker]

He raped her in the shower -- that's a pretty convenient place for washing away the semen evidence. Or he could even have used a condom.