I think there can be a lot of reasons and each case is individual. But I think that the main reason why people come to this is our environment, circumstances in life and the desire to make money in the "easy" way, when you don't know how to do it differently.
don't talk as if you know everything mate , EASY MONEY? lol selling your body to anyone you don't even Know ? laying in bed with different people each night?
it is not about easy money but about having no choice .
Most of them don't even have a proper education to compete in regular works.
if you can get over the judeo-christian idea that sex is bad or shameful then theres really no reason against becoming a prostitute so long as you enjoy sex and can find tolerable partners.
If you also know how to starve to death , you will choose to have sex to anyone .