The opinion varies but several factors were important to this.
Firstly, despite the failure of Kaito, Miku bore much of the original Vocaloid engine legacey and was seen as a successor to Meiko. Meiko had sold 3,000 units, which was already more then most synthising software packages back in 2004 who only sold 1,000 units if they were lucky. Things that had made Meiko sell well were applied to Miku.
In addition, the fact Nico Nico Douga had recently sprung up and was becoming popular in Japan was a important factor. Vocaloid PVs, demos and songs could easily be published and gain an audience, since the site was relatively new Vocaloids got attention really easily. Miku has more songs then any other Vocaloid. In 2010, it was reported she had 22,000+ original songs out of the 40,000+ original Vocaloid songs, with the next contenders beung Luka and the Kagamine's. In 2011, it was reported she had 100,000+ songs altogether.
Once people began to use the software, finding ways to apply its uses, this furthered the popularity of Vocaloid. Vocaloid was an exciting medium to the users of the software, allowing amatuer producers and professionals to compete with each other. Within a year Miku had sold 40,000 units and was already popular. This year it was reported she sold 60,000 units.
But, Miku stayed popular because of a number of issues that followed her after her realise. Until Gumi's release we there was no Vocalod who was as easy to use as Miku was. When you factor in, that there were also very few Vocaloids in this as well its not difficult to see why being easy to use was such a big thing.
Also, English Vocaloids were barely acknowledged and when they finally got attention, were mocked for being so different to Japanese Vocaloids. Its only recently the western fandom has even begun to warm up to them as well and this has been a 3 year process. Its pretty much just from Sonika onwards that there has been somewhat a english Vocaloid ("Engloid") fandom. So the overseas fandom has mostly focused on Miku and co even now.
Add with the factor that as well as selling well, since Crypton Future Media made a lot of money they could expand their services. This gave them new mediums to sell Miku on. Hence why now we see her in PSP game format and so forth.
- She got a lot of free publicity when she was first released thanks to a handful of popular internet memes.
- Popular Doujin groups like Supercell made hit songs.
- She got the imagination of the Otaku culture
- Nico Nico Douga was young and it was easier to get noticed.
- People pushed the limits of what they could do with her software.
- Improvements for the Vocaloid 2 software made her easier to use then Kaito and Meiko.
- Easy to use in comparison to most Voclaoids, especially since Voclaoids that followed were harder to use.
- She was the only "known" Vocaloid at the time of release and Meiko, Kaito and the english Vocaloids were still waiting to be unearthed from the grave they were put in.
- Having 3,000+ pre-orders and selling 40,000+ units in the first year helped.
- she had many internet memes related to her.
- she hit the Japanese Otaku fandom interests, what with her cute sounding vocals and all.
- She was easy to use and its not until 2010 a Vocaloid as easy to use for Japanese as she was came along. The Vocaloids that followed her such as the Kagamine's produced difficult results. Even Gakup and Gumi, althoguh were realistic sounding were not so easy to use.
- She was easy to tune, she was onyl meant to sound cute.
- She has a lot of merchandise related to her.
- She has had the most no.1's in the Vocaloid ranking and didn't start struggling to stay at no.1 until 2011, when Gumi's growing popularity began to catch up on her.
- Popular producers used her a lot more then any other Vocaloid.
- It took some while for other Vocaloid producing studios to catch up.
- In the overseas, Miku's popularity is owed to the lack of knowledge of other Vocaloids. Until 2010, the overseas were still much more vague on other Vocaloids including other Japanese. Other vocaloids also were met with some discrimination, pretty much the same as with English capable Vocaloids.
- Miku has been in the news the most out of all other Vocaloids and CFM are not above heavily promotions related to her.