When HD came along it showed up the flaws of the actresses a lot more. Only the high end productions kept looking so good because they could afford all the make up and lighting and decent directors/camera operators.
Then uncensored got popular and provided cheap competition. Piracy too. It became harder to sell 6000 yen JAV blurays when they were online for free, or you could just go look at porn for free on xvideos. So there was some cost cutting done too.
Maybe the older stuff wasn't all that great either, we just remember the good ones and forget all the hundreds of really bad releases inbetween.
Then uncensored got popular and provided cheap competition. Piracy too. It became harder to sell 6000 yen JAV blurays when they were online for free, or you could just go look at porn for free on xvideos. So there was some cost cutting done too.
Maybe the older stuff wasn't all that great either, we just remember the good ones and forget all the hundreds of really bad releases inbetween.