You know Low Ki/Senshi/Kaval isn't Japanese right?
As far as bringing in these other big named guys, I'm not sure about Japanese wrestling (never followed but just know some of their 'major league' guys), but isn't somewhat like how wrestling is in Mexico where it's taken more seriously? I would think it'd be harder to get well respected Jap wrestlers to come to the WWE over the Jap Jobbers (Yoshi) due to respect and different views on wrestling. But then again throw enough money at someone and they don't need respect.
There is always that about the honour of competing in Japan and Mexico, but WCW managed to get enough of those guys to make jokes about and poke fun at during their time, so if WWE pushed them seriously I couldn't see why they couldn't get a big name or two, even if it's only for 6 months to a year.