I wouldn't say it was just for cheap hits. NAKANISHI Rina's crossover into JAV has received a lot of attention, not only because of how popular AKB48 is all over Japan, but also because of how she started her idol life as a minor, how relatively quickly it happened (less than 2 years after leaving AKB48), and also how quickly she has dominated AV rankings (Rina, with just 4 releases, is on par with Tsubomi, AKIRA Elly, and ASAMI Yuma, each with 400+ titles).
Remember, Rina is not "just another" idol-turned-AV. Very few people are familiar with words like Sabra, Graphis, NS Eyes, etc (only the male, 20-40 demographic). However AKB48 is a name everyone knows. (Although interestingly enough, very few people seem aware that there was ever a girl named NAKANISHI Rina who used to be in AKB48.)
It's not that gravure and pop idols have never turned AV. However AV debuts are most commonly limited to chaku-ero or other C-list idols. Most A- and B-list models that decide to do any adult-oriented work will only go as far as nude modeling. So someone who used to be quite mainstream to make such a big leap is quite rare. And HOSHINO Asuka is in that boat.