Most Japanese want to shake up and "change" Japanese politics (many aspects of it), but they are more or less set in their ways and just put up with the same old same old, erring on the side of no-risk taking. Then they see Americans boldly elect a new guy, taking such a "change", which they themselves could never do. So Obama represents to many Japanese (without them thinking deeply about his policies and his many personal errors) the possibility of change through people power. Their media tells them to admire this trait about Americans and their votes and boldness, and so, well, they admire it. Obama just happens to be the beneficiary.
But not for long.
Failures in dealing with North Korea have woken a lot of Japanese up.