Acquaintance gang rapes are difficult to prosecute for much the same reason that any acquaintance or date rape is difficult to prosecute. The reason is that it is very difficult to prove a lack of consent.
When a stranger rapes a woman, it is generally relatively easy to prove that there was a lack of consent. Such rapes take place in places like parks or parking garages or after home invasions. In these situations, it is clear that consent has not been given.
By contrast, with an acquaintance rape, the site of the crime is usually much less conducive to proving rape. The rape typically happens in a place where the victim and attacker have gone consensually. The victim and the attacker are on good terms with one another and it is conceivable that consensual sex could occur between them. This means that the case often ends up being based on the victim’s word versus that of the attacker or, in the case of gang rape, the attackers.
This sort of case is very hard to prosecute. Juries are often reluctant to convict solely on the word of the victim when the attacker (or attackers) also swear that they felt the sex was consensual.