one major reason is that promotion work takes ages in US/EU. I believe Rihanna is still doing promo for Good Girl Gone Bad, she was just performing/promoting in Russia. she's also on a tour.
also the costs of releasing an album in US/EU & promotion are greater than in Japan. one album per year is too fast, you don't have the time to make the most profit out of the album that way.
also japanese artists just work harder. most western artists take like few months to almost the whole year off in the years they do not have an album out. most western artists go off in interviews on "how hard they worked for the album". IMO that's not hard work compared to the 9-to-5 jobs "common" people have to work at
