It's a difference of emphasis and philosophy. Both Japan and China have officially incorporated the western New Year into their calendars.
The Chinese philosophy regarding such matters, however, was "Zhong xue wei ti, xi xue wei yong." (Chinese learning for the core, western learning for use.") Hence, China pays "lip service" to western practices, and goes its own way.
On the other hand, the Japanese ethos at the time of the Meiji emperor (the time the decision was made) was "West that West is best."