Rape kits have very little to do with the difficulty of prosecuting this type of case. Date rape rarely involves a denial on the part of the defendant that it happened; the question is whether there was consent. There tends to be three problems.
1. These cases tend to be old when the police are notified. With any type of crime, the longer the period of time from the commission of the crime to the reporting of it to the police, the less likely it is to be solved. Evidence tends to go stale and people's memories tend to fade. Many of these cases are not reported for several days.
2. Directly related to the first fact is the testing to see whether drugs were administered is more accurate the closer in time to when the crime happened. Drug testing within twelve hours is very effective, showing both any illegal drugs as well as the presence of alcohol. However, nearly all of the results show alcohol and maybe some recreational drugs. Levels on the alcohol tend to be extremely high, often double or more the legal limit for driving. Question now becomes whether the ingestion of the alcohol and other recreational drugs was voluntary or slipped in. And juries are beginning to accept that extreme intoxication, even if entirely voluntary, can negate the ability to consent.
3. The last problem, which tends to be the biggest one, is the credibility of the victim. A hooker can be date raped and a nun can have consensual sex, but a jury will have a hard time believing either. It's not fair, but it's the case with many different crimes that the credibility of the victim influences everyone's view of the crime, from the police officer to the jury.
The reason so many of the samples aren't tested is due to the case being dropped by the victim and/or the prosecutor or having pled out before the testing was done. Lack of the testing being done is very rarely a factor when the case actually goes to trial.
People who go into writing have a problem dealing with its slow pace. As a trial attorney, I find the pace of the writing world so much faster then the legal one.
Best of luck,
Jim Clark-Dawe