I may not be able to fully answer this question, but I'll give it a shot.
It was partly her time of release: during the internet booming era. Any voicebank released at this time would have become the icon of Vocaloid. Crypton themselves stated her rise to fame was because of the increasing popularity of sites such as Youtube and NicoNicoDouga. They play a major part in spreading crazes such as this, almost everyone within this fandom came across Vocaloid through the internet. This is why to say 'Miku brought Vocaloid to its popularity' is imprecise. It was not just her design or her voice.
However, these do factor into it. Her design is moe, cute and eye-catching. It combines popular elements of Japanese culture with a futuristic android style. Her teal pig-tails are very distinctive, making her easy to recognise.
Her voice was also designed to be cute, as she is part of the 'character vocal' series. This appeals to the otaku audience, who most frequent the sites like YT and NND.
Her voice overall was good quality for its time of release and easy to use. This made her a good Vocaloid for beginners, introducing more to the software.
Once she became the face of Vocaloid, she spread. When you search Vocaloid, Miku will be the first one you see, therefore many people may only know Vocaloid as Miku. It is extremely rare for a person to be a fan of Vocaloid and not be aware of Miku. Miku promotes Vocaloid, therefore she is the one people come across. So her popularity increased exponentially.
She has had the most cultural impact of all Vocaloids, so her iconic status also contributes.