Or it's just an artifact of how society dealt with historical pornography that the different genders had access to over time.
There's no shortage of textual porn for men, nor has there been as far back as I've seen porn. Penthouse Letters wouldn't be legendary and universally beloved memory if men went only to the pictures.
It seems to me that that men looking at dirty pictures is just something that society grudgingly accepted. And women reading romance novels is something that society grudgingly accepted. And the rest is selective reporting of what fit the stereotypes, and reinforcing by marketers and porn purveyors who had to carefully aim their productions and so followed 'conventional wisdom'.
If I had to hazard a guess, I'd pin the whole mess on the larger social push to posit women as the fairer, more evolved gender. Thus women are cast, taught and expected to need more than 'crude' pictures; to be better than 'simple' men.
Anecdotes being what they are, I've never seen any evidence that 'women don't like graphic porn' anymore than I've seen evidence for 'women don't like videogames' or 'women don't like sport'.
And feeding my theory above, I also note that the adjusted positions of those who have accepted women gaming and women sport is that women are, again, better, more advanced and more pure than men. 'Women want more from a game than shooting aliens', 'Women athletes have better fundamentals', etc.