Given the shambles which existed that night, evidence could very well have been lost by a variety of means. With so many apparently intelligent people running around surely one of them could have suggested everyone vacate the scene of the alleged crime until CSI had an opportunity to assess it or was it a case of...
* too drunk to think
* in shock
* afraid to interfere
* just hoped she had got lost
* none of the above
I believe there did come a time when Gerry tried to keep people out of the bedroom John.
I really don't expect people who have just been catapulted from a completely relaxing situation into every parents worst nightmare - i.e. from one absolute extreme to the complete opposite - to even begin to think like policemen in the first 10/15 mins or so. The shock would be immense - and with so many people involved an initial 'shambles' would be inevitable IMO. Also of course they
would search every inch of the apartment - who wouldn't? However I do expect policemen to know better. After all it is their job.
Whether evidence was destroyed or overlooked we don't know - but it is possible. Also IIRC there is evidence which it has not been possible to rule out as having no bearing on the case. Partial fingerprints, etc.
Because evidence was not found does not mean it was never there.