Police work is a weird profession. There are cops who like working murders and those who like working robberies. Each type crime is different with their own unique challenges. There are not a lot of police officers or detectives that would say rape cases are their favorite cases to work.
There are so many issues that come up when investigating sexual assaults. One major issue that puts cops off is the issue of victim credibility. You don't often have to investigate a victim's credibility in a murder. You do end up having to look at a victim's credibility in sexual assault cases. If you don't have this thoroughly nailed down before you seek a prosecution, the defense will certainly bring the issue up at trial.
There was an interesting piece over a Slate magazine that looked at why some victims of sexual assault act in inexplicable ways, ways that may make the victim seem less than credible.
In the past decade, neurobiology has evolved to explain why victims respond in ways that make it seem like they could be lying, even when they’re not. Using imaging technology, scientists can identify which parts of the brain are activated when a person contemplates a traumatic memory such as sexual assault. The brain’s prefrontal cortex—which is key to decision-making and memory—often becomes temporarily impaired. The amygdala, known to encode emotional experiences, begins to dominate, triggering the release of stress hormones and helping to record particular fragments of sensory information. Victims can also experience tonic immobility—a sensation of being frozen in place—or a dissociative state. These types of withdrawal result from extreme fear yet often make it appear as if the victim did not resist the assault.
The whole piece is worth the read. Hit the link to read it.