The Halifax case hinged on consent. If proper consent was given then there is no crime. Period, end of story. The complainant was extraordinarily honest and said she could not remember if she gave consent or not so there is no evidence that whatever acts occurred were without consent. 
There are only two other ways to get a guilty verdict after that:
The 1st is to say that she was in such a state that she could not legally give consent. That would require knowing her condition at the time. The last people to see her said she was under her own power, talking and making decisions on her own. So no proof she was unable to give consent.
The 2nd would be if she passed out and the sex continued after that. No one can know if that was in fact what happened since she cannot remember and there were no witnesses at the time of the sex.

Before putting anyone in prison for years you need to be very sure they're guilty. The court wasn't that sure and acted appropriately.