Well, too many developers seem to be used to pump out games of proven franchises. That's not just a japanese phenomenon, but some popular japanese franchises are among the titles that show the lowest amount of improvements/changes for their sequals - the biggest offenders of this have for a long time been sports games though.
Another problem is that japanese devs seem to lag behind western devs in using the new tech and in creating content with that at a decent pace, which is why several projects seem to have lost a lot of their brilliance in level design and story design, because resources had to be used to get the visuals up to par with western productions.