Hatsune Miku is mostly contributed to this.
Early Vocaloid2, she grabbed attention because of her cute voice and avatar gaining 3,000+ pre-orders. After release her popularity was spread on sites like Nico Nico Douga via Internet Memes. By the end of her first year, she had sold 40,000+ copies and over the course of her existence sold a further 20,000+ bring the total of sales to 60,000+.
Added with this is the release of popular songs by Supercell.
She herself continues to get about 1,000+ a month on Nico Nico Douga.