In the Western Vocaloid market, it is normal for a male and female voicebank to be produced to give users a choice of voice for the same intended genre. This helps maintain a level of balance for their market, however western Vocaloid fans often take particular attention to note just how many female Vocaloids there are in the Japanese market. The theory for this is often contributed to the early Vocaloid failure of Kaito, who received few sales compared to than his counterpart Meiko at that time. Even though Kaito now sells quite well, even for a Vocaloid voicebank, Crypton began their approach the Vocaloid 2 era favouring female-based voicebanks and other studios who later joined the Vocaloid scene also favoured females over males.
As pointed out, Len, the first male Vocaloid of the Vocaloid 2 era was sold along side a female mascot and was voiced by a female singer. Gakupo's voice had been chosen from Gackt, an already famous and popular singer in Japan so good sales were expected just on this alone. Hiyama Kiyoteru has a voice that many fans felt was much the same to Kaito's voice. Big Al owes much of the usage within the Japanese fandom to his favouring of the deep octaves.
A notable point is that most of the Otaku fandom in Japan was male based in the early days, who will desire to work with female Vocaloids such as Hatsune Miku. While female producers do exist, there were not as many to begin with as there are now in Japan. It speculated that female producers are more likely to use male based Vocaloids such as Kaito. As part of the male-Vocaloid poor-sales belief amongst the studios, the first of the VY series VY1 Mizki, was referred to as having a female voice when it was first made public despite part of the intention of the series was to be "genderless".