Because they have not had the same impact as the Japanese Vocaloid did. However, the opinion that they "suck" is considered nothing more then a personnel preference. Furthermore, they have a different direction to their Japanese counterparts and do not have the same musical influences or audience to sell to.
As a note, not all of the Jaapense Vocaloid are are successful as Hatsune Miku and it is sometimes easy to believe they have all done well for themselves. Likewise, English Vocaloids are selling well enough to be kept in developement and most vocals have done well enough, with seemingly the only real failure in the Vocaloid2 developement being Tonio, who was actually rated ironically one of the better vocals.
Many little things, however, have held them back. The English language is harder to synethise, English Vocaloids contain larger voicebanks and therefore take longer to produce, English fans are quicker to judge them before trying to like them. Futhermore, most producers using them do not announce they are whereas the Japanese Vocaloids have a framework of Producers and support in place thanks to Miku's support. Cultural divide and reluctance to accept the new technology also hold them back.
Plus also take into account that at this stag with now 4 languages being developed, it is a little silly to pick on one language that has already seen a great deal improvement in its life history.