Way, way back, western(US) porn actually had story lines. In the last 15-20 years though, they have thrown away any story line, and just get to fucking. Looking at European (NOT Eastern European) and Japanese porn, they still have story lines. Sex is sex, fucking is fucking no matter where you are. What makes a film/movie/video watchable is the story it tells.
I get that porn is a massive industry, and growing. Most porn from the US is the cheap, wam-bam-now-off kind of thing. Making a porn film/movie/video, while no where near as expensive as a standard one or even a shoestring budget indie one, it is still, in porn industry terms, expensive. So the US decided to go down the easy route to money, and make the shitty gonzo et all, films, that has a decent chunk of the market.
The next problem is that porn is everywhere today, because of the internet. Amatures put themselves out there, without a big studio to help them, and are making money. There is still a market, quietly buying/downloading/streaming films with a storyline.
Porn with a storyline is much more enticing and captivating than gonzo. The most powerful sexual organ is the brain. Stimulate it, and you'll take of like a rocket.