Ask Lemmygrad

A large reason for it I'd imagine is how deeply intertwined the porn industry tends to be with triads/gangs and other reactionary groups in the underbelly of society. The PRC spent decades dealing with the massive mess of corruption left behind by the RoC and have only really gotten things to stabilize in recent years, the PRC views anything remotely linked with corruption and internal instability as a huge no-no.

From what I seen China is really not that heavy handed against erotic stuff in practice as they are on paper, if anything lightly erotic stuff is all over the place in China. It's more of a strategy of making things inconvenient enough to keep the masses away from it rather then any real intent to arrest everyday people for looking at porn.