www.rawstory.com/2016/10/porn-sites-block-users-from-california-to-protest-condom-ballot-measure-report/
They claim it's because condoms cause microabrasions. So best not use them.
There is some speculation in the article that actually the industry doesn't want to open itself up to lawsuits.
I also wonder whether there's a fear that men won't want to watch porn where the actor is wearing a condom aka showing some consideration to the risks run by his partner
Sex work is just a job like any other, eh?