I don't know.
There are lots of reasons. For some it is because they, at least temporarily, feel wanted. For others it is the high of sex and/or the thrill of danger being with someone they don't know. For some it is just a bad decision, or maybe not even a conscious decision, that has been fogged by drugs/alcohol. For some it is because they want sex but not a relationship.
Assuming this link is intact, you can download a power-point presentation that studied your question from cornell university that is pretty interesting: