New ideas are hard. Old ones aren't! No wonder there are so many sequels. You've already got brand recognition and a built-in audience. According to Ajay Chadha at Mercury Meltdown publisher Ignition Entertainment, Japan is totally afraid of creating new IPs. They are risky! Chadha says:
A lot of Japanese publishers are commenting about coming into and developing in the Western market, but I don't think that a lot of Japanese companies are taking as many risks as they used to... You've got Devil May Cry 4, Metal Gear Solid 4. No one's really taking the risks with original IP. They think the answer is to come to the US or come to Europe and found development studios which would appeal to the West. I don't think that's the answer. People are just not taking that creative risk anymore.
He does think that Suda51's No More Heroes is a risk that will pay off. Though, I don't think the unwillingness to make new, risky IPs is unique to Japan. Do you? Japanese Game Makers Fear [Next-Gen]