Those two things seem to be separate enough. Not only is there a very blurry line in between the two, but also creation of the disallowed content is much safer to do there than to apply to a shady studio to get yourself out there.
So the second question would propably be why is porn disallowed in the first place? Is it the payment processors who don't want to deal with chargebacks? Something to do with investors, perhaps their own "morals"?
As to why violence and war is more acceptable I believe the answer lies in that intimacy and explicit content is much closer to home, its relatable, it attracts feelings and is also easily accessible.
On the other hand war, violence and gore are really a far away concept that does not resonate with a lot of people. Its also an established ok-thing so it being all around us in the form of entertainment makes people even more numb to the subject.
So the second question would propably be why is porn disallowed in the first place? Is it the payment processors who don't want to deal with chargebacks? Something to do with investors, perhaps their own "morals"?