You realise there were awful Japanese Vocaloids right?
The Kagamines, for example, have a Act 2 for a reason and Miku herself was dated by the end of the Vocaloid 2 era compared to later Vocaloids being released. She wa popular because she was the first to get attention, to be there at the right time to get that attention, but was made to sound cute; not realistic. This continued with all members of the CV series the Kagamines and Luka. In contrast, Sweet Ann and Prima were based on professional singers and produced realistic result, this is why it was common for people to presume in Japan that they were higher quality then their own Vocaloids, because Miku and co were awfully robotic sounding to them.
Also consider there is a reason for Gackpoid - Native, Lily - Native, Megpoid - Native, VY1v3.
Also, note that Voclaoid 2 was incapable of usingTriphones, whereas Vocaloid 3 was capable of using them. Therefore there were sounds even the Japanese Voclaoids couldn't seperate due to their lack of distinction (rival freeware software UTAU, however, already had them in 2009).
Its also worth noting you can more easily pick faults in languages you understand then ones you cannot. Hence why lots of people ares unaware of what impact Triphones missing from Voclaoid 2 have, nor how bad Kaito and Meiko's pronouncation of Japanese was.
So really, it just boils to do opinions, perferences and language. If Japanese was just as complex, there would be no difference in quality. If you want you should listen to Spanish, Korean and Chinese Voclaoids and decide if you can tell how good the quality is for them.
Most people base so much of their opinions on persumptions, which is why Gachapoid gets rated quite low quality at times. There is a reasdon why many PVs contain lyrics in Japanse Vocaloids uploads... Because they can barely understand them as well sometimes.
And note, just becuase you like a song, it doesn't nessecary mean it was good or well tuned, or was a good use of Vocaloid, many popular songs are actually quite awful. Secondly, for songs that are well tuned; don't mistake a well tuned song for a good Vocaloid, producers have made some really good songs using some really bad Vocaloids, and vice verse there have been really bad songs for even the highest quality Vocaloids. And a a lack of songs does not reflect a Voclaoids quality either; for a long time both VY1 and VY2 had few songs despite being higher quality then Miku.
And finally consider HakuYowane was made to represent awful Miku songs orgiainally which is you don't believe any of this is proof that not everything from Japanese Voclaoids is nessecary as great as people think.