Why is porn illegal in china when prostitution is not, or atleast the laws not forced?

Simply because prostitution is a source of income for some Chinese people. The average income for some people in China is like a 100 dollars a month for long hours and maybe 1 or 2 days off in a week. A good looking girl with no trade skills or qualifications take come to the cities and earn maybe like that much for a few hours work.

Porn on the other hand is not. The average policeman on the beat doesn’t really want to arrest this follow country person because they are only trying to make a living. The odds are they will shoo them away so he doesn’t have to deal with them or turn a blind eye or they will a cut in money or favours. The only time is when the government want to do something about it or make a statement. Given the nature of this and its history in China its seems more socially ok to

Porn on the other hand does not have to involve Chinese people and can be seen as an outside corruption and national politics may come into play here.

Also with porn giving physical evidence it would be a lot harder to convict people under the law to prove that x and y where hooker and customer than a person who was caught with a hard drive full of it.