not an expert, but i can roughly guess why, based on certain things.
- Japan society, as many of us know, is still pretty conservative. Any loud or brash noise/outburst, esp. from the females, are discouraged. So making and watching porn is an act of rebellion/deviance against sociatal norm.
- speaking of vocalizing, there's something called Female copulatory vocalizations. Japanese females, by tradition, are expected to be gracious and reserved in any emotions. Having the JAV actress loud moaning and vocalizing is breaking that general traditional expection even during sexual acts.
- Many japanese females naturally have voices in the *higher register, which japanese males traditionally likes. As to why in almost 90% of JAV, they either raise their voices' register even highter, are squeaky, or even sounded as if tortured... beats me. Probably based on the past sales that these sell especially well...?
*a note, is that i remember reading an article from an issue of Times magazine's Japanese Culture Special edition. It summise that Japan especially loves Youth and Youthfulness.
the above are just my personal thoughts on it. feel free to point of any mistakes.