In order for a crime to be committed, you need two things: Actus Reus or Guilty Act, and Mens Rea or Guilty Mind.

In "alcohol rape", assuming there is a clear guilty action (rape) and no question of consent, there is still the absence of Mens Rea. Intoxication is a valid legal defense. In the eyes of the law, the person who committed "alcohol rape", if they were drunk, was actually temporarily out of their mind, and is therefore not fully responsible for their actions. How were they supposed to know what they were doing, they were Drunk!

This does not mean the victim of "alcohol rape" doesn't feel violated, often far from it. But no matter how clear cut it may seem to the victim after the fact, in the eyes of the law, it is not right to punish the perpetrator with a full sentence.

Now, in a sample case between two intoxicated people, who are both emotionally involved in the case, and therefore biased, and no outside witnesses, Proof beyond a reasonable doubt becomes almost impossible to establish. When the stories don't agree, or worse, are fuzzy on the details, perhaps changing with multiple retellings due to alcohol's effects on memory, how on earth is one coherent version going to be established? A case like that is very uncertain, and the most likely verdict is "Not Guilty"

Now, it becomes even more complicated if consent is in question, which it usually is in these cases. Perhaps the person wouldn't have consented under different conditions, if they were not drunk, for instance, but this could hold true of both parties, and is not provable. Did the person really consent? Was it informed? Was it obtained under force? Perhaps they were misunderstood, or changed their mind later. All this is hard to determine from a hazy retelling.

Naturally, we feel deep sorrow for any victims of alcohol rape, but because people are Innocent until Proven Guilty under the U.S. legal system, and we require a proof beyond a reasonable doubt, these cases are almost impossible to prosecute, and are seldom as clear cut as the victim's assertions.