Since forever. There are a few reasons why - one is cultural the other is risk. On the cultural side industries that are "controversial" tend to be avoided. Payment processors don't like to be associated with porn, gambling, drugs etc. because of potential negative PR. If you're a payment processor and trying to land big conservative clients, having press about how you support the porn industry doesn't do you any favors.

The other side is risk, meaning industries that have an above average chargeback (fraud) rate. Many times these industries overlap - porn for instance. Lots of stolen credit card numbers are used to by porn, but also legitimate purchases can easily end up as a chargeback. The classic example is that someone buys porn on their credit card and then their partner finds out. Instead of admitting they bought porn, they claim someone must have stolen their credit card to save face.