From my perspective, the most significant difference between two countries lies in the underlying thoughts toward the guardianship.
In case of Japan, it seems to me that a number of experts and laypersons consider that the guardianship (especially plenary guardianship) itself contributes to protect vulnerable aged persons from various kinds of obstacles.
On the other hand, it is deemed that the system substantially deprives them of individual's autonomy (the right of self-determination) in the US. For the sake of preserving their capacity, as the fundamental concept of guardinaship in the US it must be the less-restrictive protective measure.
In fact, not only UGCOPAA (the Uniform Guardianship, Conservatorship, and Other Protective Arrangements Act) but also state legislatures themselves encourage courts to consider other least protective alternatives to guardianship firstly before appointing guardians unlike Japan.
Considering above, the difference related to each judge's authority may have more or less influence on this issue.