It's because cocaine dealing is an enterprise that they aren't controlling... you're pulling large amounts of their money behind their backs. Understand that the judicial system has two parts; control, and keeping society content. Anything the judicial system does is either to keep society content or to better control the people to the will of the biggest business in the world.. the U.S. Government.
You understand that when they print money and give it out to you it means nothing but a piece of their power, and when something like drug trafficking comes along and moves large amounts of their power into unwanted hands without their control then you're fucking up the U.S. Government's business, it's like embezzling money from the business. The government loses billions every year to drug trafficking simply because of this, you're moving large amounts of their money into unwanted hands without their control.
Rape sentencing simply falls under the "keeping society content" category of law enforcement, the reason it is so much less than a sentence involving dealing cocaine is because the U.S. government doesn't lose billions of dollars to rapists like it does to drug trafficking, in fact rapists don't take any of their money so they are of a much lesser concern to the U.S. Government, the only reason they even give a rapist jail time in the first place is to keep society content.
It's that simple, it's about money and the U.S. government is a very cost-effective business.