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The English Vocaloids don't 'suck'. They're much harder to use, however, and they aren't as popular, so there's a significant lack of good songs using them.
Take note, it is harder to use a Japanese Vocaloid for English. while it true English Vocaloid overall harder to use, they are not THAT hard. If a user types the words "I love you" they will sing "I love you". There are now as of August 2011 over 600 English Vocaloid songs but there are literally thousands of Vocaloids songs that have been produced dwarfing that number, the quality and user skill of which vary per each song. Out of every thousand songs, only a handful are of good status, but the opinion of "good" quality makes it difficult to judge the exact number. With much less songs, it is harder to find one that is on par with what fans perceive as "good".
Take note, to say the Japanese Vocaloids are better is not entirely correct and even they have flaws. One of the most common problems fans have when comparing Vocaloids from Japan is the acknowledgement that they are listening to a language that they may not even be able to understand to begin with. Unless you have an understanding of phonetics for the Japanese language you will never be able to fully "hear" the Japanese Vocaloids. However, a native English speaker can hear clearly how flawed the English Vocaloids are in contrast.
The level of skill a producer may use in making a song may also be deceptive; even the worst Vocaloids can sound good in the right hands. For example, the Kagamine Vocaloids have had a long history of problems with their voicebanks, yet it is rare that anyone will notice this within the English speaking community. Even Hatsune Miku herself is a dated voicebank and improvements in the Vocaloid 2 were applied over time with VY1 and VY2 being the highest quality voicebanks produced for the software. Even then they are still not perfect, yet reading comments on youtube about the two Vocaloids people have responded with comments like "most realistic Vocaloid ever". While this is true, they still cannot tell how far they are from sounding realistic and "human" like.
English Vocaloid usage has only just begun to increase and many are still trying to learn how to use the software. The amount of development in the English version and its community is still lagging behind because of the software early bad start and its reputation. As usage increases more development will go into the software. However, in its early days the English Vocaloid software was doing much better then the Japanese software at a time when it was a obscure software package few had heard of and without Hatsune Miku's popularity no one would have heard of it even now.
Even still, the fact many fans regard the English Vocaloids in less light is proving to be its biggest problem it faces. Many expect Miku's English voicebank to be perfect, yet it will still have the same flaws and strengths as other English Vocaloids. Fans who hear Japanese Vocaloids singing in English often are left with the impression the level of capabilities they have in English are higher then they actually are. The reality is that it takes many long hours to get a Japanese Vocaloid to even sound remotely like its singing in English and there are many tricks they may use in the process to perfect this result. Consider that very few fans realize Luka's level of English is rated only at a sub-standard level while Miku's will be "of standard level" of English.
Though there is a note that the Japanese language is phonetically much simpler (1,300 samples are included per each Japanese Vocaloid) than the English language is (8,100 samples are included on average in English Vocaloids) making it harder to maintain a good quality result for each English Vocaloid. However, English Vocaloids do have a greater language capability owed to the fact they have a wider set of phonetics to choose from and have far more pure vowels and are capable of producing a much larger array of single consonant sounds.
With new languages on the way and a slow steady increase in English Vocaloids, the perception may change in time, for now, it is advisable that fans give them breathing space to expand and develop upon. The Vocaloid software itself is still quite young with many years of development ahead. Regardless, the bias opinion (for instance the title of this question alone is implying they suck to begin with) is the biggest factor in how they are perceived. It will take some time for fans to warm up to them, and even longer for them to realize the truth behind them. In Japan, however, the fans acknowledge their unrealistic and inhuman results and accept how they sound.
- see Phoneme List