Not only do they prefer the underdog, people (including myself) find that the Japanese pair has more appeal (feel free to disagree) even though they are undoubtedly weaker than the Chinese pair(s).
Watanabe has a weak smash but what he lacks in smash, he more than makes up for in other departments - especially with his flashy plays (sometimes); this alone makes his matches so much more watchable, exciting and entertaining. Also, Higashino's defence and soft touch is weak but what she lacks in those, she more than makes up for in athlecticism and speed, which most women XD players don't possess; not to mention her jump smash (the form and technique) that brought her fame in the earlier years.
Zheng and Huang play textbook XD and they just do it clinically with consistency and hence they are way above everyone's league. Zheng smashing like a machine vs the skilful and artistic movement of Watanabe? The choice is clear for me.