Why isn't vocaloid an anime?

Vocaloid is not an anime because, well, vocaloid is a program made in Japan where you buy the characters and the sets and you can make your own videos. Sometimes they're fan-made and other times they're made by the actual people. So, to answer your question, Vocaloid is a program, not an anime or manga.

Vocaloid isn't an anime, but they have made a manga of it. It's called Hatsune Mix, and you can read it on onemanga.com.