There are many reasons why people go into prostitution. Some possible reasons are:
1) Being physically abused as a child
2) Being sexually abused as a child 
3) Being involved in drugs
4) Prostitution isn't always a choice though. Sometimes people go into prostitution due to being forced.
5) Sex work also be a lucrative business depending where you are from. Those are not academically capable may choose such a practice as a means to make more money.
So despite the sexual drive, the main reason for prostitution in all groups is money.