The evidence given (and unchallenged, as far as I can see) is that McDonald asked if Evans could join in and the victim consented. They both corroborated each other's account. The victim couldn't challenge it as her accepted evidence is that she can remember nothing at all of what happened.
To prove rape, the CPS woukd have required to demonstrate that:
That the victim was not consenting (because she was incapable through intoxication);
(ii) That Evans did not reasonably believe victim was consenting.
Evan's evidence was that Macdonald asked if he could join in and the victim agreed and that, further to this, she was an enthusiastic participant in what followed. Again, both Evans and Macdonald corroborated each other and the victim could not give any evidence to challenge this.
For the avoidance of doubt, I think that Evans behaved in a pretty despicable way. However, this is a very unusual case as it is fairly rare for the victim in a rape case to remember nothing and be unable to challenge the evidence of the defendants. Further, every case does turn very much on its own facts - particularly rape cases where the determination of consent is obviously key.
There are a lot of good points on this thread but there is also a lot of mis-reporting of the facts and misapplication of the law which I think is perhaps unhelpful in respect of women deciding to report rapes and sexual assaults.